I have a 37 and 39. I would have a 38, but ever since the 3rd gen finger grooves, the compacts don't fit my hand well. I love this round. It's got the ballistics of a .45 ACP, with less recoil and in a grip the size of a 9/40. I decided after the first time I fired my 37 that it was going to become my new carry caliber. I'm still in the process of switching over; I've been slacking.
As far as ammo, on the mainstream ammo sites, it can be a challenge. I've found that Georgia Arms has it in stock pretty consistently and the prices aren't bad at all. http://georgia-arms.com/45gap.aspx
Considering that there are 5 state police agencies (NY, GA, PA, SC, FL) that have adopted the caliber, and from what I've read/heard there are other smaller agencies that have adopted it plus others looking at it, I don't think we have to worry about the round fading away. Speer seems to be handling the ammo needs of those agencies, so I don't think any of the other manufacturers are making it in the kind of volume that Speer does. It would not surprise me, in the least, if other agencies pick up the caliber over the next few years and it continues to grow. IIRC, NYSP was the first agency, in 2007, to adopt the G37 as their issue gun, so even though the caliber has been around since 2003, it's only been in the last few years that it's started to take off.
45 GAP is about the same ballisticly as 45 ACP +P, it came about because of LE's request for a big bore round with a smaller frame due to the number of women getting into LE who couldn't properly grip a full size 45. Good round but not the magic bullet some make it out to be.