Is Tropic Thunder corny? It kinda looks that way....would you recommend I rent it?

It's probably a good choice for a rental. It's entertaining and does have some really funny moments. It also has a decent amount of action (explosions, etc.). I am not a fan of cheese ball comedies and I saw Tropic Thunder in the theater expecting to hate it. While I was not blown away, I didn't regret it either and I laughed out loud many times. It has a few goofball moments that I could have done without but for the most part it held up as a fun movie. It's definitely worth seeing if only to satisfy your curiosity - I don't think you'll want to build a time machine and go back to not watch it after all.