Megamek is OK, but way too buggy for normal play. The interface is pretty difficult, too. My friends and I tried to set up a multiplayer battle a while ago, but it took so long to set up mechs, install the game, learn the controls, and figure out the game interface that we gave up after the game crashed or dropped a player for the umpteenth time.
I hope MegaMek's developers take some time to improve the interface and the network stability.
Mechforce's interface is much nicer. All of the commands and windows were fully labeled and accessible through mouse or keyboard, and the mech building tool was really easy to use, so newbies to the game could pick it up really easily and games could get set up quickly. The only big problems with Mechforce are 1) I have to drag out my Amiga to play it (I don't using the emulators), 2) my friends and I have to play it in hotseat mode because there's no network play, and 3) the movement and attack are time-and-turn-based instead of traditionally turn-based. Although it's not the same as the official rules, the third point is actually a good feature of the game as well, because it made a heck of a lot more dramatic to see mechs like Locusts and Fleas charge out of forests toward assault mechs that couldn't turn fast enough to respond.
I played Titans of Steel: Warring Suns (
http://www.titansofsteel.de/), which was a recent remake for Windows of Mechforce, and though I like it, its interface has too many obscurities for it to be a pick-up-and-play game.