Here's my review cut/pasted from another board - I'd put it in its own thread to not jack this one, but it seems this has become the official 5150 III thread. Surprisingly pretty similar opinion to jabps and simbasa above -
I spent a good deal of time with the 5150 III halfstack in a demo room turned up pretty loud, using a custom shop Fender strat, a Les Paul custom, a Les Paul classic, and a Dean explorer with Dimebucker pickups. In short, it sounds very tight, but kinda flat to my ears - I kept wanting to get more girth out of the sound. The character is like the 5150 (not the 5150 II) but smoother, less harsh. The first channel is a good clean, not the best. The second channel does some great plexi sounds, great definition and shine with no saturation. And then channel three is the improved Peavey sound.
I was a bit disappointed with the humbucker guitars through it,. The Les Paul sound spectrum is already compressed compared to a lot of guitars, with most of the sound in the lower-mid range... the amp didn't seem to help "fill out" the LP tone, if that makes any sense. The strat on the other hand sounded fuller in comparison through it, particularly in channel 2 playing non-saturated rock tones.
I'm using the Mesa RK, MkIV, Diezel VH4, VHT UltraLead for comparison. For playstyle I like heavy crunchy rhythm tones and juicy shredding lead tones. After owning the amps listed above, I can't say I felt tempted to buy a EVH III.
So there's my 5-cent HarmonyCentral-ish review - Consequently another guy was in the room watching while I was playing; he was blown away and bought the amp right after I put the guitar down. So *shrug*..... take my opinions with a grain of salt.