Well, I'm a pretty satisfied customer of my digi001. I love the mastering tools, but to be honest I don't have a lot of experience to compare it to.

The midi between 4x and 5x is night and day, so don't let that scare you off. The midi sequencing is good, just not terrific (such as it lacks groove quantization or event editing). The audio editing is superb. I love being able to shape my panning with a square wave or sinusoid and have it locked to a tempo.

One nice feature is the bus tracking so that when you get some tracks laid, you can save the original audio and then bus them together to a new track (applying any plugins at this point) and then freeing up those tracks.

One problem with the digi001 is that it is very proprietary. Supposedly the projects in the digi will convert to a TDM system, but I've heard this only works on the Mac at this time. It will probably be added in the future to the PC, as well.