I'll second this. Beyond Pac-Man Collection, Robotron 2084 and two-player competitive or co-op on games like Centipede really make it worthwhile. The 7800 is all I've been playing lately.
As far as game recommendation goes, for the 2600, most of the classics have already been listed, but I'll reiterate with a solid list of Amidar, Berzerk, Circus Atari, Demon Attack, Jungle Hunt, Kaboom!, Kangaroo, Keystone Kapers, Megamania, Millipede, Missile Command, Phoenix, Pitfall!, Pitfall 2, Pole Position, Riddle of the Sphinx, Solar Fox, Solaris, Spider-Man, Turmoil, Venture, and Yars' Revenge.
For the 7800, check out not only the homebrews, but also Asteroids, Centipede, Dark Chambers, Dig Dug, Food Fight, Galaga, Joust, Mario Bros., Pole Position II, Robotron 2084, and Tower Toppler.