I always see a huge boner for PPI Art Series amps, but after over 10 years of using them myself, I know that they aren't the end all be all of amps. I've had A600.2s and an A600, as well as an older PPI Sedona amp. Sound quality is great. They look cool. At least at 4 ohms, they never miss a beat or get too hot.
But their terminal design is lousy. Worst I've seen. The little plugs are easy to lose and expensive to replace. The gold plated metal in them is extremely soft. The screws get stripped easily and they just don't hold onto wire very well. Considering the power and ground wires are right there next to each other, and can slide out pretty easily, it's downright dangerous. Those who have really used these amps in a few different installs will know exactly what I'm talking about.
I had the same Art Series boner for a long time and refused to upgrade my amps. They finally started to crap out. Dead power supplies, bad RCA inputs, etc. I'd pay a LOT to have them repaired, only to have them go out again. It's like once they start to die, they really start to die. Finally, back in '06, my truck was broken into and they stole three A600.2s and the Sedona amp. I was devastated, but replaced them with three JL Slash amps. I really, really like these JL amps, and they haven't given me any problems at all. I had an A600 in the shop when that all went down, and got it back shortly after. I use it to run my front mids, and it works great. But as soon as it craps out, I'll replace it with a Slash amp, too.
In all fairness, I can't say that Slash>Art Series, but that's only because I haven't used these amps for 10 years to see what kind of longevity they have. But as far as today is concerned, I'd rather have a new Slash amp than any amp that's a decade old.