Everything or almost everything has been said already. No need to add much, except that mine (bought secondhand to an AF user) has two jj tubes and came with the pedal that goes with the jmp1.

UTILIZATION

The configuration couldn't be easier. Personally, I connect it to the return of my fallen angel head.
"Is it easy to get the right sound?": oh yes!!! Especially since everything is configurable on the jmp1: clean sound, distortion sound, bass, mid, treble, presence, effects loop level, gain, volume.
Regarding the patches available, 26 are already preset by the manufacturer but they can be modified at will. All others are entirely free for you to create your own sound.
The manual is simple enough to be easily understood by anyone. A table details the 26 preset patches (volume level, gain level, bass level...).

SOUNDS

Connected to the loop of my fallen angel, I have tested the jmp1 with my 3 guitars: an American strat, a gibson les paul smartwood and a ltd viper tuned one semitone down.
I "reproach" my fallen angel for having a too sharp, too solid, too modern sound. I want heavier sounds, so that's why I chose the marshall over the engl 530 or the voodoo valve. And I was not disappointed, on the contrary. For people who love rock, in every sens of the word (poop rock, hard rock, heavy metal), this preamp is for you. The sound palette is such that it's impossible not to find the sound, or sounds, you are looking for. I haven't had it that long to be able to really exploit all the resources of this beast.
For those who think you can't get a big and fat sound with the jmp1, I can guarantee you that you are wrong: with an EQ (mxr 108) in the loop, even an overdrive to boost everything (mxr wylde or ibanez tube screamer), and the possibility to adjust all parameters, we can go from a superb crunch to a very nasty and fat sound (maybe the jj tubes have something to do with that? I haven't tested it with the original tubes, so I can't confirm that).
It's true that the jmp1 isn't the best for clean sounds, but it wasn't made for that. Nevertheless, with a bit of reverb and a compressor we can get very nice results.
Finally, something that came as a really nice surprise to me is that it is possible to get fat distortions at low volumes with this preamp. I suddenly have no use for my vhm power break anymore, which was indispensable before (I've put it up for sale).

OVERALL OPINION

My opinion might be very subjective, given my little experience with amplifiers, but the jmp1 truly corresponds to the sound I was looking for and that I like so much. You may criticize the marshall sound, but nevertheless it has something and it's part of rock history. The jmp1 has reconciled me with marshall because I was very disappointed after trying out a tsl 60 combo which paled against the fallen angel.
I highly recommend this preamp given its great value for money, especially secondhand, if it's in good condition, like mine (thank you aliascross!!!).
What I like best is the diversity of sounds and their configurability (is that even a word?)
What I like least: maybe the fact that you only have four sounds available with the footswitch (but then again, a midi pedal will help you easily get round that problem).

Edit from 01.06.07: after several days I changed my jmp1 for another preamp (an engl tube toner). But it wasn't because I was disappointed by the jmp1, but rather that I had no use for one of the preamp's main advantages: the configuration possibilities it offers. The jmp1 requires a lot of time to fine tune the sound, which is something I don't have. That's why I took the opportunity to change it for a tube toner that has more basic settings and is easier to configure. I still think that the jmp1 is a very good product, and who knows I might end up coming back to it someday...?