SPOILERS BELOW for anyone that hasn't seen it.
I agree. Looper had potential and the first 20-30 minutes had me engaged, but ultimately, the ending was exactly like Donnie Darko. Joe has revisited the loop we see in the film many times before ... how many, we don't know. Hundreds? Thousands? He's been presented with the position to change something at the end when his older self (Bruce Willis) shoots Cid's mom which causes the downward spiral of Cid becoming evil, closing loops, and propagating the cycle to continue once again. And this cycle will forever continue until Joe intervenes and causes Cid to become good and not develop into the Rainmaker. Joe is the sacrifice to all of mankind much like Donnie Darko. If Looper was written such that Joe WAS Cid then I think we have a winner as a badass time-travel film.
If you haven't seen Primer, it is at the apex of the genre. It was written by scientists. Low budget and only 80 minutes long, but it's one of the most intelligent time travel films that exist and one that offers something most plausible. One of the writers, Shane Carruth, collaborated on Looper with Rian Johnson. It is confusing as fuck and you will likely have to watch several times before you get it, but it's an amazing movie.