We moved our 3E campaign to C&C when 4E came out, as I needed a break from the increasingly cumbersome 3E system as everyone leveled up. It hit the sweet spot for me, with its broad compatibility with everything from OD&D through 3E (and probably 5E), but I never stopped having to puzzle through the Siege Engine, which isn't nearly as clear as the Troll Lords think and is an obviously subpar resolution system. But I loved that we could grab monsters, spells or even whole character classes from other editions of the game and throw them into C&C without breaking stride.
Their class design is great and the class-and-a-half system lets groups create unique classes that feel like 1E classes should have included. A balanced, spell-less ranger and spell-less paladin by default (and a knight class, for folks who want their paladins to be entirely free of supernatural influence) are great, and you can then add back in spells through the class-and-a-half system. My campaign had a ranger with druid spells and one who had cleric spells through her worship of the goddess of home and hearth, and they both had a distinctive feel to them. Likewise, C&C gives you the 1E assassin, which you can turn into a 3E assassin by doing class-and-a-half with illusionist.
Some of their supplements, which have gone through multiple systems and have been converted by people without real system mastery (Book of Familiars, I'm looking at you) are kind of a mess, but it's the excited puppy enthusiasm of early Dragon magazine articles, rather than a cavalier attitude about putting out good products.
They've also put out a really solid line of various cultural takes on C&C (for the most part -- the Germanic and Norse books have a large overlap with each other and probably should have been one book, rather than asking people to buy the same material twice), something I'm surprised other major/second-tier companies haven't emulated in the same way.
The whole line could use a serious edit (at one point, their two main monster books had competing versions of Zork's grue, which no one at Troll Lords headquarters seemed to have realized) and I could not care less about their official setting that they increasingly focus on. But the Troll Lords themselves seem to be just about the nicest guys in the roleplaying business and their love of early to mid 1E radiates through the whole line (as opposed to Goodman's focus on early 1E flavor).
I would love to see someone come in with a ton of money and help them upgrade their line to more professional polish and maybe pry the Siege Engine from their fingers in favor of a more standard D20 resolution system, which I think would make their game instantly more accessible.
