Our group
In the campaign I'm playing in, we have 6 players all playing gestalt characters. I believe they consist of Human Cleric/Monk, Dwarven Cleric/Samurai, Human Fighter/Barbarian, Human Fighter/Rogue (think Pirate), Gnome Rogue/Wizard, and, my character, Druid/Monk with a Vow of Poverty (only the greatest feat ever). We're all 5-7 level, and we're playing Return to the Temple of Elemental Evil.
I'm on my third character. The first, a monk/rogue Indiana Jones for the Temple of Azuth packed a hell of a sneak attack. He died in a player Vs. player conflict (a whole different thread). The Second was originally Sorceror/Bard and at 6th level replaced Bard with Fighter to increase his BAB (I was trying to qualify for a broken prestige class.) He died last session trying use his rapier instead of his magic missiles. (That's what I get for playing in character.) I haven't really had a chance to play the Druid/Monk, but I figure he'll be pretty powerful.
As a player, I find that the versatility is great. It really seems to allow you to play more unique characters, because you still have to explain in-game why you can do what you do.
My GF played a fighter/rogue as the daughter of Inaigo Montoya from the Princess Bride. Fighter/rogue is much better than the Swashbuckler class at actually reproducing the Cinematic swashbuckler. And that's really the reason why we like the Gestalt system. It creates a far more cinematic system, which is what the DM likes.
The power level, at 6th level, is about right at +1 ECL. I suspect that at higher levels, the ECL will have to rise to match the accumulated powers. But for now, +1 ECL is right. As I said above, the characters are not overpowered.
The most powerful combos are those when the two classes share a main stat. To really break the system, you'd need Uber stats. For instance, Sorceror/Wizard is hardly playable unless you have 18 Int & 18 Cha. If you're using, point buy, you now have no points for Dex or Con, both of which you'll need. If you're rolling dice, good luck. I would think the odds of getting 2 18's on 4d6 - lowest is 1 in 1,000. Sorceror/Paladin is pretty good, but you either worry about Spell Failure, or you use no armour, or you just don't use the paladin's fighting to it's fullest. Monk/Cleric seems to be powerful but not outrigt broken. In fact, I'd say that it's one of the more powerful combo's but it has yet to prove to unbalancing. (You face a similar quandry as the Sorc/Pal, the beneifit of the Cleric is the armour which you can't use as a monk.)
I'm hoping that Monk/Druid is a powerful combo. We'll be playing with Complete Divine starting next session. I hear there will be a feat that will allow druid's to cast heal spells spontaneously. IF that's true, I really will have a broken character.
Just my 2 platinum,
Eric