I only just started rewatching some anime due to 8 or so successive seasons with very few new shows worth watching.
It has been interesting. Rewatching Cowboy Bebop, Berserk (though it has definitely aged since I last saw it!) and Baccano! was very rewarding and solidified my feeling that they some of the best shows around.
On the other hand, I also tried rewatching Elfen Lied...and I just couldn't do it. You'll find that you won't necessarily like every show you may have enjoyed in the past. The cutesy parts of Elfen Lied drove me friggin nuts. Originally I liked the contrast, but now I can't stand it.
In the middle were Samurai Champloo and Basilisk. Still enjoyable (particularly at the beginning), and my memory *isn't* particularly good, so a lot of it seemed similar, but i didn't recall all the details. Still, I didn't like them nearly as much as the first time. Particularly the latter.
I'd say it is worth doing if you have the time and don't have much new to watch. I also liked Blood+ and think I will try and watch it again. Hell, you can simply stop if you can't get into a show.