I've seen hybrids in play, though not played one. There are actually a lot of workable hybrid combinations, but there are also a lot of traps:
MAD (your hybrid classes must have compatable stats, or be able to safely ignore a stat).
Actions (your classes must not both need your standard action to function, and it's dangerous for them both to need your minor action to function; this can be partially be mitigated with sufficient synnergy or if you're content to play a modal character)
Implement/weapon compatibility (This isn't actually that hard, as you can multiclass into a Holy Symbol or weapliment class and use all your Implement powers without needing to switch weapons or spend a hand. But if one class uses two dagger, and the other uses two longswords, things might get awkward).
Armor compatibility: Combining a heavily armored class with a lightly armored class can work out very badly. Sure, you can spend a feat and your hybrid talent slot to get your armor back, but now you're not getting any other optional features.
Even with all that, it's not hard to come up with cool builds (beyond obvious stuff like paladin/warlock, wizard/swordmage, the cheesy rogue/ranger based around immediate actions and minor action attacks, and rogue/warlock(Assassin)). Leaders and some defenders do all their stuff with minor (or free) actions; controllers, fighters, and most strikers do all or most of their stuff with their standard actions (except that controllers do need minor actions to sustain--but they're ranged so they don't always need to move). So fighter/leader, controller/leader, striker/leader, defender/controller, and defender/striker will tend to be places to find hybrids that are more than the sum of their parts (as long as the have compatible stats, armor, and weapons/implements).
I suspect Shaman would be a more popular hybrid if you couldn't get almost everything cool about being a Shaman with a few feats.