DoctorSpuds

Unlike some other consoles (looking at you Colecovision), the 7800's usually work fine.  Save2600 identified some possible problem points... it might also be worth mentioning that their shells are a bit more brittle than most other consoles and should be shipped with extra care.  (I've had a few shipped to me that ended up in pieces!)  

As for the games, for me, it's all about the arcade ports.  Food Fight, Galaga, Robotron, Asteroids, and Xevious are all great ports.  The Donkey Kongs (Sr and Jr) would be good if it wasn't for the sound.  Besides the ports, I personally rather like Ballblazer.  After that... well... there's a lot of mediocrity in the library I'm afraid.  But the homebrew scene is excellent, particularly thanks to the efforts of "PacMan Plus" (Bob DeCrescenzo).

Failing Pain-line controllers (buy and Edladdin Sega converter) and a somewhat finnicky cartridge connector.

If you want to play the original cart versions (not on a harmony) of Robot Tank, Space Shuttle, Activision Decathlon, you will likely have to snip out the capacitor at c-64 (right side of the motherboard, between two smallish ICs.). This was added in most revisions of the 7800 to allow the 2600 version of Dark Chambers to play, but resulted in making the above Activision games unplayable. Removing the small green Cap at c-64 fixes this, and also makes ti compatible with the starpath supercharger.