I was just curious who else carries a back up piece. I do at times, usually my Keltec P3AT. I have the mindset that 2 is 1, and 1 is none. I usual do carry my BUG concealed though, if someone does try for my OC piece a BUG gives me more options in that situation. I could also do the old "New York Reload" which in some cases might be quicker than a standard reload. There is also the possibility my primary could fail or get shot (it does happen). I also think that BUG could also be used to arm another person in rare cases.
I think the best BUGs are the S&W 642, P3AT, but there are certainly others that are equally servicable. Of course nothing beats carrying the same model pistol as your primary or a good longarm if available. A close second would be a pistol the accepts the same magazines or ammunition as your primary, like a G23 and a G27 or a full sized 1911 and a commander sized backup.
My BUG (and occasional primary when the situation dictates deep CC) for a year or two now has been a S&W 340PD .357 that I carry on my ankle. It is a bit painful to shoot with many full power loads, and I have the Tim Taylor disease that won't allow me to "settle" for .38 Special +P loads, even for a backup. On the plus side, it is so light that I regularly forget it's there.
I just ordered an ankle holster for my G36 .45 ACP. Like the .357 Magnum, the .45ACP round is adequate for backup and occasional primary carry purposes. I recently found a brand of pants with large enough legs to allow the G36 to be drawn reasonably easily, yet don't look too "teenager". If the added weight and size don't make ankle carry of the G36 uncomfortable, I may find myself carrying that one more often.
I normally prefer revolvers, but mostly because I haven't had a choice. They don't make a semi-auto round with the stopping power that I want (see above referenced TT disease), so big-bore revolvers are what I've become accustomed to. That being said, there are some advantages to autopistols vs. revolvers, and the G36 is much more pleasant to shoot than the 340PD. I'll post a follow-up after I get the Ankle Glove for my G36 and wear it for a while.
Personal preferences (revolver vs. auto, caliber, how you plan to carry your BUG, etc.) will influence your decision, just like the choice of a primary weapon. I'm just sharing my $0.02.
You could always buy a second of whatever your primary is and wear it weak side as your BUG.
I'll probably carry my G27 along with my G23 from time to time. I need to pick up a good ankle holster first. We'll see what I can come up with. I just ordered over $300 worth of leather. My wife is gonna kill me if I buy another holster just yet.