I generally use rechargeables and have been for a long time now. Since the introduction of NiMH batteries, we've seen the mA-Hr ratings go up from 800 to 2500 for most AAA-D cells (good alkalines generally have a nominal 2400 mA-Hr rating), but the "square" 9V cells have stayed at 150 mA-Hr, just like their NiCd predecessors. The 9V NiMH batteries charge better but there are some things to consider:
(1) They lose charge in storage. Some say that you should figure 1% per day. My experience is that the 9V batteries lose charge in storage at a faster rate.
(2) If your alkaline battery gets you through three gigs without replacing, then you'll be able to get through a gig on a 9V NiMH. My experience is that they drain over twice as fast as an Alkaline for most pedals, wireless transmitter, tuners, etc.
(3) You should always charge a spare for each applied battery. Sometimes you get a false charging. Sometimes you get an inexplicable drain.
(4) Always have spare alkalines. Sometimes you don't remember to get the charging process organized in time.
Paj
8^)