Sounds wise this unit is excellent. I own Mesa/Boogie TriAxis, Mesa/Boogie Studio, Soldano SP77 Series II, Bogner FISH, Groove Tubes Trio and Rocktron Piranha preamps. All of these are just straight ahead tube preamps with the exception of reverb on the Mesa Studio. In comparison to these units I would state that the Marshall has one of the largest pallettes of sounds. I play primarily a PRS CE24 Maple top guitar with the preamp going into a BBE 362 sonic maximizer and the into a few different power amps then into two 4x12 cabinets with Celestion Vintage 30's. Utilizing the same power amp I would say that the Marshall is definetly the closet to nailing down the Marshall sound, yet it doesn't come accross with the umph or balls of a Marshall head(I have two JCM800 50 watt heads). I believe this to be related to the actual voltage the preamp tubes are running at. The Rocktron Piranha has much more punch and bell like tone to each note. When a preamp runs the tubes at low voltage it seems like the preamp is running with a govenor on it and if you throw alot of notes at it they seem to smear together. This may be the sound you are looking for so each to his own.
From the day I bought this unit I had a problem with unit. It would not change programs or the LED would just flash. I tried reseting the unit per Marshall instruction manual but no luck. I called Tech Support and spoke to John. He was great. The problem seemed that my unit obviously been sitting on a shelf for a few years and the back-up battery CR3032 was going dead. To change it out I would have to send it to Marshall for repair. Guitar Center had no more in stock so I couldn't exchange it. I figured that I would do this myself. Well let me tell you that they do not use a standard battery, it is a CR3032 but it has two legs/connectors soldered to it. It was a test of patience and cordination with a desolder and soldering iron. After about 1-1/2 hours and a trip to Radio shack I was in business. This is a key thing to remember because if you have you customized programs in this unit you better back them up or at least right them down so you can replace them when the battery dies your up the creek impaled on the paddle. I have used it on gig without back-up. The tube looks to be mounted in a very delicate manner, you can easy rip the tube socket right of the motherboard. The tube socket should have been molded to engage with circuitboard or chasis so the stress of the tube is not transmitted to the board. A similiar arrangement as to the lamp end sockets of a flourescent light fixture would work. I don't know if there is any telltale sign to when your battery is going to die so reliability is somewhat of an issue. Most preamps that are programmable at least you can dump in the factory programs via system reset and finish a gig, but not on the JMP-1.
It's a good sounding unit, adjusting the sounds are little menu intensive seeing that there is only one data wheel. I know there must be a million of these models out there and I haven't had anybody I personally know have that much negative things to say about it. I prefer a different preamp for gigging so I leave this olne in my home studio.