I've trie a few in the past, and generally not liked them - partly because they can sound dull/aggessive and partly because my Pbasses have always had a J added and it just makes a bad balance worse!
Having said that I really liked the old DiMarzio, has a nice mid edge to it, and really liked the Seymour Duncan trad version. Its also worth noting that stock Fender pickups can vary a lot...so overwinding isn't such a new idea. As the good Doc says, the after market sellers need to show an improvement..however slight..over the original.