As far as fantasy goes, Wheel of Time isn't one of the best series, but it also isn't anywhere near being one of the worst series. Unlike certain other authors I could name, Jordan actually does evolve his plot substantially (as opposed to rewriting the same plot over and over) and for the most part refrains from descending into black-and-white moralism, which is one of the reasons I feel his series works more often than not.
The biggest problem is that his writing can be slow at points, even painfully slow. "Taking a paragraph to describe the hem of a character's dress" slow. The worst offender is book ten, which mostly takes place in the span of a couple of days and retells the same events from most of the major characters' perspectives; Jordan later admitted that this was a failed experiment. However, the early books in the series are much better about this, as are the three most recent books in the series (book fourteen will almost certainly follow suit, because, well, it's Sanderson).
The other big negative point is that his characterisations aren't really that great. A lot of his characters have similar traits and behaviours (braid tugging, for example), and some of them can be extremely frustrating at times. However, for the most part they're nowhere near as bad as WesLY says they are.
That said, I think the strengths of the series outweigh the weaknesses. Jordan has a fairly strong grasp of the complexities of political intrigue, and his world-building is quite strong. The level of detail in WoT is close to that in LotR, if not even more so (Jordan didn't invent an entire language, but the huge span of the plot means you get to travel the world in much greater depth); you really get the sense that this is a world that has existed for thousands of years, with its own cultures and history. He also formulated one of the more interesting magic systems in fantasy literature, which not only relates to the plot but even ties in with some of his themes.
Most of today's "epic fantasy" owes a huge debt to Jordan, as he was really the first modern author of mega-sagas to reach a wide audience (granted, Brooks and Donaldson preceded him, but they started a long time before Jordan, and their scope doesn't seem to have been nearly as wide as Jordan's), so those interested in the history of the genre will likely find it interesting to read his works because of how many people built off his themes. This is not to say that other authors have not done more interesting things with the genre since - some of them include Terry Pratchett, China Miéville, and George R. R. Martin, in my opinion, although Pratchett is a contemporary of Jordan's and Discworld does not really count as a mega-saga in the same way WoT does - but often times one's appreciation for the genre deepens when one has a sense of its history, so I'd recommend it to anyone with a serious interest in the genre. Also, despite the length and massive number of characters, WoT doesn't actually take that long to read, probably because of the huge number of descriptive passages alluded to above. None of the volumes took me more than about four days apiece to read. Granted, I didn't have much better to be doing at the time.
That said, I'd probably recommend putting it off until the last book comes out, so you don't forget half the plot in the interim. I'm waiting until 2012 (it's anticipated in the late part of that year) before I start my re-read.
As for Sanderson, I've read everything he's written solo for adults except The Way of Kings (which I have on my shelf for later) and do not regret doing so. First off, he's one of the more perceptive authors of fantasy to emerge in the past decade or so; he does not boil down issues to simple questions with simple answers, and his works are stronger for it. His magic systems are pretty much as good as you can get in the genre. In some ways they are pretty much the opposite of Jordan's, in which anything can happen, because they're so intricately diagrammed out that fans of, I believe, the Mistborn series were able to correctly puzzle out what the rest of the system was before he had even revealed its complete formulation. And the other aspects of his world-building are solid as well, and his characterisation and plotting work well. The one weak point is that his prose doesn't stand out that much, but it's hardly bad. Well worth reading.