The way the system worked was pretty neat. The effects of being a sidekick were linked to the main character, or "mentor" -- the combat level of the sidekick player was artificially boosted to one level below his mentor. If the sidekick wandered too far off, the power boost would also wear off. Sidekicks received experience and rewards relative to their actual level. In other words, it wasn't really a system for speeding up the leveling process, so much as it was a way for players to play with friends, regardless of whether they'd just joined the game or not.
Sure, you wouldn't be as powerful as your mentor, but it would give you an opportunity to play with them as you leveled, rather than trying to level off in a corner at a breakneck pace that, frankly, isn't a lot of fun. This is one of those things I've always wanted to see in WoW, because it embraces what the game is about -- playing with your friends and having fun, no matter what level you are or where in Azeroth you happened to be.
City of Heroes may be gone, but the sidekick system was one of the really shining developments of that MMO, and it's a system that would be silly to let go to waste. What do you guys think? If given the option to play with your max level friends, would you rather take a potion and boost your XP gain so you could catch up faster, or play at their side right off the bat?