Coil splitting simply drops the output and makes a full-sounding guitar sound thin. I've never heard it work well. Re Philly's comment re the PRS DGT, it is probably the best coil-splitting setup that there is. I'm fortunate enough to own one and I can tell you that the coil splitting sounds awful even on a guitar of this awesomeness. That's why I'm selling it - a Les Paul or 335 with BKP's makes me sound like ME, so happily I can stop looking for tone! The good news is that if you want a Fender-y sound you can buy really good strat's and tele's quite cheaply and put BKP's in to take it to a whole new level. I think Tim's expertise really adds value to guitars.
Sometimes it works, sometimes it doesn't. With the A-bomb on my strat, it actually works quite nicely. I was playing with a clean tone, and the split humbucker gave a really really good tone there. Might have been luck, I have to really test it at stage volumes where I've rarely used it..yet. At least on its own.
I don't really care for it on the Riff Raff bridge that's on my PRS McCarty. That's where it feels somewhat thinner. However, the middle position on that guitar gives very nice tones on the split position too. And full humbucking mode. I might try the parallel setting on this guitar though!
One setting which I often use on the LP, is the middle position, full bridge humbucker and the neck humbucker split. That's where it works good too..
-Zaned