I don't see much point in going on too much about the music in this collection - it's all Britney's previously released singles with some remixes and b-sides. You'll have heard all these songs before and either you like Britney enough by now to consider buying a greatest hits package or you don't. The girl's been around 10 years, we know what she's about by now!
The issue, I feel, is that although this is a nice way to repackage everything it's a pretty expensive one, particularly if you're a long term Britney fan who is more than likely going to already own these songs. While I don't think that's much of a barrier to a standard Greatest Hits CD, when it's a big expensive package like this it's a problem.
Namely, I think the problem is who they're aiming at. The best market for GH packages is the casual fan - they don't own every album, but like the songs they hear on the radio so to have a Greatest Hits which cuts out the filler is ideal for them. These fans, however, are unlikely to pay through the nose to own an artist's stuff and so this boxset is unlikely to move them (unless they're canny enough to ask for it as a Christmas gift). The kind of fans who are likely to pay a lot for an artist are the hardcore ones... but then these are precisely the fans who will already own everything she's put out and don't really need to spend so much on a boxset for the sake of a solitary new track. They'll already have the b-sides and remixes from the individual single releases.
Some people will always want to have every release for the sake of a full collection - if that's how you're looking at it, then this really isn't a bad package and if you tot up how much you'd spend on buying every single release individually, it probably works. But it's a lot of money to drop in one go and to be honest, if you're not a hardcore fan looking to complete the collection then I'd suggest you're far better off buying the standard edition than this box set. I love Britney, but this really isn't the most economical way of buying her stuff.
So in conclusion - I would suggest to most people that this boxset isn't the best way for you to buy this and you'll be better off buying the standard CD & DVD edition. You'll get fewer tracks, but it'll be more value for money.