It's a tough question, because it's so hard TO seperate the game from the controller. I honestly couldn't imagine playing the game on anything other than the controller it comes with.
Is it fun? Yes. No question. It's one of the better mech games I have played.
However, there are a few of provisos there...
1) It's not a Gundam-type mech game. Don't expect to start pulling off fancy stuff in these... they are big, lumbering, they can tip over if you aren't careful, be knocked over by an impact, run out of fuel, stall, overheat, you name it. These things are called a vertical tank, and the name fits. It's VERY important you understand this before you buy the game, IMO. The game is very fun, but it's not paced the same was most mech games are. If you go into this expecting a game like Starsiege or Mech Assult, you are going to be disapointed. It's more akin to the feel of an old PC game called "Battledrome", really.
2) Expect to suck at first. Learning to co-ordinate the multitude of functions on the controller takes a while. Don't expect to strafe or dodge or anything much at first... concentrate on staying vertical and hitting your target.
3) You really have to have a good setup to use the controller on... a card table, for example, placed in front of the TV.
4) I wouldn't try to play it on an exceptionaly small TV... 29" would probably be the minimum I would suggest... You might manage on a 21", but I wouldn't wanna try it.
5) This is a big one. If you don't have a memory pack to back up your games on, keep the eject button uncovered. You don't want to die. If you die (That is, your mech is destroyed and you don't eject in time), your pilot is killed... which means your mission progress is lost. It doesn't delete your saved game (Like some people say), it just moves the pilot to the KIA roster, and you can't play him anymore.