Felix's always seems crowded and a little hectic. You can generally seat yourself, but let someone know you are there if you are sitting outside. If you are sitting outside, make sure you are ready for the sad, gothic kids outside of Diedrich's to blow smoke all over you while you eat. yum.
There is a huge assortment of food here, so I can only speak to the few things I have had. I think that they give you soup or salad with your meal, I seem to remember having soup. I've had some of the seafood dishes which were nothing special. I've had a chicken dish which was good. They have this spinach mixture as a side which is tasty, but not healthy in any way. It's like spinach drenched in alfredo sauce. The sweet plantains seem authentic, but I don't like them as a rule.
Black beans and rice are always good and for the low-budget students, it's a very good and cheap option to get to go. They give you a ton.
They also give you (for dinner at least) tres leches cake for dessert. I really like their cake and i'm not a huge tres leches fan.
It's all pretty cheap for what you get.
Some people love this place and consider it an institution for Cuban food. I don't think it's that fantastic, but it's pretty good every now and then. Maybe I just need to find my dish there.