From my own experience, I would recommend strongly against it.
When you are holstering it, or while it is properly holstered, there is a strong chance that your pistol will be pressed against the holster and—Oops!— there goes your magazine.
I've seen it happen, and more than once.
It is much more useful to learn, and then to practice, proper use of the normal-length magazine release.
I have em' on several guns... I like em' and have never had an issue. I always check my holsters to make sure there is clearance. Putting one on a Glock is like night and day. I have never dropped a mag unintentionally.
Try it out... if you don't like it, you can always switch it out. Only takes a minute or two on most guns.
As for CCW, an extended magazine release in a good hard holster with clearance maybe, but in a soft IWB holster I've experienced un-intentional and unknown magazine release a time or two which was very uncomforting when I found out. In this case I don't see the benefit of an extended magazine release outweighing the possible risk. I don't want to come up with one shot thinking I have 15 nor if I had a magazine disconnect coming up with none.
I once had one on one of my gen3 Glock 23's. I filed the serrations down to make the contact point smooth and I rounded the corners some to improve the feel. Then one day I was carrying this gun in a leather holster and when I got home and removed it, I noticed the magazine was not fully seated in its well. I removed the extended magazine catch and re-installed the factory OEM unit.
Most always I carry my Glocks in Kydex holters and this problem doesn't occur in them. If you get a gen4 Glock, you most likely will not want an extended magazine catch because the one in the gen4 is larger and you don't have to twist the gun in your hand to press it enough to remove a magazine.