Any time your engine rpm is below the stall range on a transmission that has a non-locking torque converter, you are generating heat due to the slip in the converter.
This can be particularly bad if the vehicle is very heavy or used for towing and/or you're driven in hilly areas.
Example: 3500rpm stall converter in a street/strip pickup truck. Someone decides to hook up the 24ft boat and tow it to the lake. As you pull the grade in traffic, the engine is at 2000rpm and the converter is slipping. Trans heat builds without bound...toasted tranny in no time flat.
So what you want is something with a stall speed that isn't too far above your normal cruise rpm, and ample transmission cooling. A gauge is recommended so you can keep an eye on trans fluid temp.
I drive a blown 1993 Lightning truck with an E40D with a locking 3000rpm stall converter. If I program the PCM for no lockup, my trans temps will easily hit 200deg F or more on the freeway within 30 minutes of flat cruise. If I enable the lockup, the transmission rarely gets above 160. And, this is with significant front-mounted coolers in both cases.
Personally, I am a huge fan of ATI and doubt I would use another. Cost was a bit of a factor but when I can think back it was eiher '93 or '94 when I bought it, I can't complain at all. I gave them all the info they asked for and they sent me the converter they said I needed. I have been happy ever since. In a 3600# car it stalls around 3000 and I've gone as numerically low as 3.25 gears with no trouble. With 3.00's I experienced the problems Byron spoke of. I do not have a temp gauge so in my mind the trans temp is always perfect. Lee