I have an Umarex Glock 17 which has a rifled barrel, but comes with mags for both BBs and 177s, although I suspect putting BBs through it would not do the rifling any favours, so with that thing I stick to pellets, and it is undeniably good fun and is pretty accurate as well, so I've no problems in recommending it, if you like Glocks. But bear in mind that the one I have - Umarex make lots of different variants with some quite significant differences - is not field strippable, whereas some of the others they make are strippable, so if playing with it and having it able to be stripped down is something which appeals to you, then pick one of them which is strippable. Personally that didn't bother me, so I was happy with the one I got which is not strippable, but on the plus side, it does come in a nice hard plastic Glock case similar to what the real thing comes in. Also bear in mind that the one I have does not lock back on an empty mag, whereas some of the other Umarex Glocks do, so this too might concern you if you want a 'just like the real thing in every way' CO2 Glock.
Here's my Umarex Glock 17, which if you like Glocks, does undeniably look the business:
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Having said that, in spite of their popularity with law enforcement people and civilians alike, real Glocks and indeed license built airgun replicas of them are not for everyone in terms of ergonomics. Since the Glock 17 is a pistol with a double stack magazine to enable it to hold 17 rounds, this makes the grip pretty wide, which some people don't like. Also note that Glocks have a moulded in thumb slot on the grip which is geared toward making you grip it with your thumb low down on the handle and as you probably know, that's not typically how most professional shooters recommend you hold a pistol, instead saying you should use a high grip and place your thumb along the frame just below the slide, which then leaves space on the grip to allow you to have the end of your other hand's thumb on the grip, then wrap your fingers around the grip with your second hand's thumb going along the grip just below your other thumb. So for most people, that thumb recess is a somewhat annoying feature on a Glock.
Now of course the 'recommended' two-handed grip is to give you good control of recoil and follow through and to put your dominant arm as close to being in line with the barrel as possible, all of which aids accuracy and avoids 'limp wristing', which is where you aren't braced enough to allow the slide to properly function, which can cause a failure to load a new round when the action cycles incorrectly owing to backwards movement from your wrist (a problem with semi-automatics which is not limited to Glocks incidentally). This is more of a problem with a real firearm than it would be with a CO2 blowback air gun since the blowback on a CO2 replica is basically 'just for show' and to give it a small amount of kick, but since part of the appeal of such a CO2 replica is to use it like the real thing, it does kind of matter if that is part of the appeal of such a gun if you want to use it as per the real thing for example to replicate some tactical competition shooting or similar.
Then there is the fact that a Glock famously does not have a safety catch in the traditional sense, instead featuring a trigger within the trigger which sort of acts as one, but it's not everyone's cup of tea and many prefer an actual safety catch, so if you are one who does, this is a big negative point for a Glock replica, or at least it would be if it slavishly followed this aspect, because having said that, the Umarex Glocks do actually depart from the real things where this is concerned and do have a recessed safety switch under the barrel area (it's very well concealed so it doesn't spoil the realistic look). But it is kind of hard to operate if you have short nails, however it is there and a usable safety feature, although if you then decide you want a laser or light or some such on your Glock mounted under the barrel, this makes that safety catch inaccessable.
These are all things you might want to consider where realistic replicas of Glocks are concerned. But if you like Glocks, don't let this put you off the Umarex ones; the fact that they share the same issues that the real ones have, are license built and have all the correct markings etc, is actually a testament to how good they are, warts and all.
Some other recommendations if it doesn't have to be a Glock...
Sig Sauer as you probably know are currently experiencing a lot of problems owing to a well publicised incident (when a US airman was allegedly shot and kil;led by his gun going off whilst in the holster) wherein their P320 model is susceptable to negligent discharges caused by the play in the slide allowing the striker mechanism to fire the pistol without actually having to touch the trigger. Again, this is a problem with the real thing and not the airgun replicas of them, but if you like replicating the real world, you might want to consider that, if you wanted a gun which was popular, because curently Sig is not, at least in the real world.
A CO2 pistol I can certainly recommend for a bit of fun and I'm certainly not alone in doing so, is the GAMO P25 Blowback, and that's for a couple of good reasons: It is one of the more affordable ones, but it is in spite of that, very solidly constructed. Although not an actual replica of a real world gun, it borrows heavily from the looks of the Walther PP (that's the bigger original Politzei Pistol, not the smaller PPK of James Bond fame) so it has a classy old school look to it, but if that look does not float your boat, you can 'tart it up' with a dedicated 'tactical kit' available seperately (but this only costs about 20 quid) which will allow you to throw more tactical rails on the thing than you could ever possibly need and which makes it look very much more up to date
Here's mine, with that tactical kit on it although note that I also modified mine so it could take a silencer, so the regular ones don't have that threaded barrel or that silencer although GAMO do make the PT85 Tactical, which is internally similar to the P25 yet has all kinds of tactical stuff that comes with it, including fancy sights, silencer, torch attachments etc, so it kind of looks like my modified P25:
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If however, you don't want a Glock and don't want that GAMO because it isn't a replica of a real world firearm, then I'd recommend you take a look at the Air Venturi replica of the Spingfield Armoury XDE. Like Umarex replicas, this is a licensed copy made with the approval of Springfield Armoury, so it looks just like the real thing, has all the correct markings, weighs exactly the same as the real thing and it is fully field strippable, but in spite of all that, it only cost 130 quid, and in fact if you look about you can find it for less than a ton in some shops, which makes it an unbelievably good bargain and something a little different from all the other stuff out there.
If you aren't bothered about the blowback feature but you still want a gun which has a real world counterpart and is probably the most accurate-shooting one of the entire lot, then I'd recommend the Umarex Walther CP88. This comes in various versions, but they all shoot well and being non-blowback, are very miserly on gas. The downside of course is that whilst they look like a real Walther, the magazine system is nothing like the real thing (being the Umarex rotary type, which technically makes it a revolver pretending to be a semi-automatic), but if you can live with that, you wont find a better shooting replica.
This is my nickel plated one with the compensator attachment, which isn't to everyone's taste, but it is certainly the best-shooting replica pistol I have and it's reliable too. I've had my for getting on for 20 years and it still shoots well:
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You should be awre that it's a slippery slope, and you'll end up with several of the things, but you probably alread know that.
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As for pellets I've only tried JSB Diablo and the lighter JSB Express. I hoped the lighter pellets would improve the low shooting, but it didn't.
