If you can get a good mixing board, those tend to have the best pre-amps, but most of us don't have the money. I've been blessed enough to have employment at a commercial studio in town, and enjoy classic warm pre-amps on a 16-bus AMR console. If I were running a commercial studio, I wouldn't ever want to do without a good board. But just using an interface is alright for home recording. If you can actually afford a Neve or SSL though, then you're foolish not to go with that!
I'd say in general, if you want to spend less than $50,000 on your recording gear, then do without the board. Any more, and not using a board is silly, since if you spend THAT much, you probably have clients, and you need not only the best pre-amps, but the convenience of a board in order to keep sessions running smoothly. Paying clients don't like to pay for wasted time.
At home, I just have an M-audio firewire 1814 with a digital octopre, and it's great for all my home recording projects.