I love Fire & Ice on the corner of Berkley Street and St. James Street (one block from Boylston Street). It's about $17 in the evening, and it's all you can you. You pick out the meat, veggies, and sauce you want, and then bring it over to the grill for the chefs to cook. The food is great, and if you make a combination you don't like, just get something else. I recommend taking small servings so that you can try lots of things. There's also one in Harvard Square, but I like the one in the Back Bay better.
I also like P.F. Chang's on South Charles Street in the theatre district. It's part of a chain. The food is great.
If you're looking for something a little bit nicer, the Hammersly Bistro on Tremont Street in the South End is excellent. It's a bit pricey and has a rotating menu.
I've never been there, but I hear that Jae's is really good.
Out in Cambridge near the Galleria Mall, there is a great new restaurant called Bambara in the Hotel Marlowe. Excellent food.
Karen