In short, it is a practice that should only be undertaken by persons very very familiar with the particular line of dogs in question.
LOL - of course, this can be said about any breeding strategy!

Even with outcrossing, you run a chance of doubling up on recessives; the difference is that linebreeding creates homogenity, which means you're more likely to see double-recessives in the first generation, where outcrossing creates heterogenity, which means you probably won't see double-recessives until a few generations down the road, but the single recessive can become more widespread overall. So, different risks. You definitely don't want to linebreed exclusively - that's where the problems start, when you don't bring in "new blood" every few generations. An excellent article on the different breeding strategies is here:
http://www.mbfs.com/compuped/bell.asp
"You breed the best to the best, and hope for the best."
And that's really what it all boils down to, whatever 'type' of breeding you do.
