If u go on the M-Audio boards under the USB forum, at least every other thread is a problem with the fast track series. I just got the MobilePre MKII with MP9, and I use an ISA One as an external pre.
I used to have the old MobilePre and just got the new one. I can tell there's a difference in sound quality, but with the MKII, there's some weird things going on with how I have to gain stage the ISA One/MobilePre. I'm going line in, but its almost like the MobilePres preamps aren't getting bypassed or something.
Anyways, that's my weird experience with it so far, but I haven't seen anyone else with this problem so don't let it discourage you. At $150 the MobilePre/PT MP9 is a cheap safe way to get your feet wet. That's my 2 cents