I have a 424mkII, and I think it's great. In fact, I have two.
The 424mkII is one of my favorite Tascam Portastudio designs, mainly because of it's grey color and it's flat design, with top mounted XLR's and I/O connectors. That top-mounted connector design is a large part of what's great about the 424mkII.
I produced a major piece of work on the 424mkII called "Dave's Ramones Tribute Album", which I think is "pretty darn good", and a pretty fair example of hifi recording on the 424mkII, if I do say so myself.
I guess you'd have to hear for yourself to know what I mean.