The best story telling I've ever seen!
It's incredibly original. I've watched this movie a few times to try to understand it better each time I rewatch it.
I just noticed how it makes us feel the same way Lenny does. For each session, we don't know what happened before, as Lenny also doesn't, and everything seems odd and out of place.
Everything just starts from nothing and each time we see Lenny struggling to figure out what's happening, until the end of the session when we get the clue of what was happening before the previously shown one.
Providing this feeling to the audience is only possible by splitting the movie into blocks of session and presenting these sessions in backward chronological order. The movie wouldn't be this fun and we'd not feel what Lenny is feeling if we weren't as lost as him.