Mbox is apparently an excellent piece of kit. Pro Tools LE is a capable and industry-standard piece of software, though I don't like it. If you decide you don't like it, either, you are not 'locked' into using it, either.
Other similar products (some firewire/USB and I think some being PCI with breakout box) include:
Mackie Spike which includes my sequencer of choice, Tracktion. Also has on-board signal processing which takes some of the load off the CPU. Tracktion 2 is almost out, but I don't know what the details are for its inclusion with Spike. If it DOES come with Spike, it includes hundreds of dollars of industry-standard plug-ins as well, and a full suite of mastering plug-ins from Mackie/Acuma. Also, a Mackie-specific version of Ableton's excellent Live software.
M-Audio's FastTrack budget solution, or their awesome Firewire 410. M-Audio's stuff always comes bundled with software, too. Ableton Live Delta and Reason Adapted for starters, in most cases.
Edirol's range of products, only one of which being the UA-25.
Tascam's US-122 which includes the highly capable Cubase LE, as well as a reduced version of the industry's most powerful and ubiquitous sampler, Gigastudio 3. A word of warning, though-- at higher gain levels, the US-122 has the reputation of having some cross-talk. In other words, if you're recording and playing back at the same time, some of the playback signal bleeds into your input. I've used the 122 personally, and I haven't noticed this at all; however, it's very well documented that it's the case, so I'd believe it. To me, as a hobbyist, it's a worthwhile trade-off for the package you get.
[slightly edited:] At the end of the day, a PCI with mixer solution offers you the most flexibility for the lowest price, but the trade-off for flexibility is plain ol' physical space in your work area, not to mention portability. Many days, I'd rather have one of those standalone devices like the MBox.
Greg