I have 2, and they're brilliant :) I had planned on having one modded for high gain stuff, and one for cleans, but there is plenty of gain on them, so i only modded the clean one... Hook it up with some of it's inbuilt Chorus, and you've got a really nice lead tone too :)
As Jonathon said, the MDRT upgrade is the best thing you can give one- that's the only thing i've done to my high-gain unit, and it made a great unit even better :)
Never played a JMP-1 myself, but i've been lead to believe that they're not as good, but can't say myself :)

I have the MP1 3TM as my high gain and have to say it's brilliant
It has an extra valve and is similar to the units Paul Gilbert used to have that had been modified
The guy that did the MDRT (Machinator) rebuilt my ADA power amp too.
Marshall JMP1s are ok but a bit limited and a touch bland.
I tried one next to the Pirhana and the Pirhana sounded more real , behaved much more like an amp and had knobs on to control it
Was a lot more expensive too (I paid nearly £800 while the JMP1 was only about £500 at the time)
It was so good I bought a second one so I could have one aht home and one at work-
I'll sell the spare for £395Here it is in an out of date piccy of the rack of doom- second from the top
I Also have an Engl and Soldano preamp in here now and am using a Marshall power amp
