The Times Online has a lovely biographical sketch of Roget, he of the thesaurus. There's currently something of a tug of war in our department about whether we should be recommending thesauri to students. Of course, there is nothing inherently wrong with them. From a practical viewpoint, however, problems do crop up.
These almost all stem from the fact that there are very few thesauri written with a language learner audience in mind. In fact, I have been able to identify only two: The Longman Language Activator & The American Heritage Thesaurus for Learners of English. The difference between these and your run-of-the-mill thesauri is that in a learner thesaurus, the words are explained and exemplified.
As the article points out, thesauri are really best used to jog loose an already known word that has become stuck somewhere in its journey from brain to hand. The typical pattern is to look up a common word (common words typically being easily brought to mind) to free a relatively rare one. For language learners, however, the lookup is most commonly from unknown (or poorly known) word to other unknown--and relatively rare--words. This is obviously not a particularly valuable process. A far better approach is to focus becoming familiar with more common words.
I'm not blaming Roget or the thesauri for this state of affairs. If students are going to use such books, it is up to teachers to help them see how to use them. Even with proper training, though, I'm not very confident that students will actually follow through. I'd prefer that they focus on using a dictionary well, ably, accurately, adeptly, adequately, attentively, capably, carefully, competently, correctly, effectively, efficiently, excellently, expertly, irreproachably, proficiently, properly, rightly, satisfactorily, skillfully, smoothly, soundly, splendidly, strongly, successfully, suitably, thoroughly, and with skill.