My friend's got an MBox on a Mac G4 and loves it ... I've used it at her house. If you know Pro Tools, it's easy. If you don't I think Pro Tools is a very logical setup and easy enough to get going with -- there will be a learning curve, sure, but I don't think it's excessive.
The USB limitation is why you can only record 2 tracks max into the MBox. It takes XLR, 1/4", and SPDIF input -- just not simultaneously. If that's not an important limitation for you, it's a pretty good deal, as it comes with recording software and usable plugins (may want to look into an outboard compressor or other compressor plugin, though).