I have gotten fat injections three times over the last 14 years for naso-labial folds. The first time, it didn't last all that long.
The second time, the doctor changed his technique and it was only about 8 years later that I decided I needed to do it again. 8 years ago, the doctor put some fat in my upper lip and it has lasted ever since and looks very natural (not a trout pout). The first time when I had fat injections, but I didn't get the long-lasting results, I was only swollen for two or three days or so.
The last two times, when I got the great results, I was grotesquely swollen (and I mean grotesquely) for about 10 days. Even after 10 days, I looked freakish to my critical eye. So, it's not exactly a lunchtime procedure. In fact, the last time I had it done, I had to take so much time off from work, I thought "well, I could have just had a facelift" for all the down time. But, if you can deal with the downtime, I highly recommend it.
Pain was minimal - only a bit of pain when the needle goes in. No pain afterwards - just grotesqueness. I think the chief advantage is that it can stave off a facelift for a couple of years if you don't yet have a bad neck or horrible jowling.