For board games, the iPad is fantastic. It can easily replace a 6 foot stack of folded cardboard and plastic bits.
For some games, like accelerometer based ones, or slower paced touch based ones that were written expressly with the iPad in mind, again, the iPad can be great. A lot of that depends more on the programmer than the iPad.
Unfortunately, for hardcore "gamer" type games...FPS, fighting games, heavy 3D graphics based stuff, etc., the iPad seems more of a novelty. It's breathtaking at times to see what it's capable of, but for long term enjoyment and playability, it falls short. The lack of physical buttons is a problem. I suppose a remote controller could help, but at that point, a dedicated gaming system is probably the better choice, from an ergonomic standpoint.
Mirroring on a TV initially held some promise in theory, but I found it's almost impossible to play any but the simplest button masher games without tactile feedback, as I was constantly glancing back and forth between the TV the iPad.