Boosting still works. I rarely boost, as i find that questing is faster, more fun, and results in more developed characters when I've got them leveled.

However, my latest group, three warlocks, two priests (one shadow, one disc) had the warlocks get two levels ahead of the priests when the locks got their pet quests finished. Two of the locks were linked to each other, the other lock was linked to both priest. "Ouch", says I

The priests were level 39, and I had a reject Lv-36, mage (in horrible gear) on the vetran account that i'd never finished. He was my very first toon, a gnome, and I found that I hate gnomes, as the camera is so low that I often couldn't see anything. Anyway, in came the little mage, and I took the RAF trio to ZF, with an unlinked lv-70 lock to boost with. Worked great, once I discovered how far back I needed to keep the mage. At level 36, the mage was an agro magnet farther than my lock could shoot :P

A couple of hours later, each RAF toon had gotten two more levels from grinding (no quests), and had caught back up with the original lock group. As the group leveled, they got more experience, but it was all RAF triple experience (Lv-36 while linked with a pair of LV-39's = aprox 450 per kill, Two Lv-41's, while linked with the single Lv-39=aprox 600 per kill each)