First off for those of you who are saying Canon needs to stop with the whole DSLR for movies thing. That was not Canon's idea. Larry Thorpe, Imaging Guru for Canon, never ever intended the 5d Mark 2 for movies. People still use them. But that's a different topic.
Ok here are the tiers that need to be conquered before we get to Arri:
1. 4k - The C500 requires some sort of external recorder for 4k. RED's been doing it a long time and IN CAMERA! The Sony F55 can at least do it internally. Yes it's compressed, but you can do slow motion internally as well on the F55. You can't do that with the C500.
2. The Cinema EOS Line works great in the HD world, but is wildy over priced for what they do. The C100 should've never been invented (before you attack me I bought one so don't hate haha). The C500 and C300 should've just been one camera that can shoot 4k and a C300 like camera should've been at the C100's price point where you can at least shoot on proper CF Cards and not annoying AVCHD with an HDMI cable (I miss HDSDI)
3. The F55 with an external recorder can give you at least 16 bit linear RAW which destroys what can come out of the C500. Not to mention the lovely 240 fps that camera can do. The RED Dragon can do 6k (True 4k...almost) and internally and shooting 5k, 4k, 3k, 2k, 1080p (junk on the RED but oh well). And up to 300 fps on the RED.
4. Canon has to compete with all that before we even touch Arri who is the established juggernaut in the industry. Personally there is one camera the will wipe the floor in terms of resolution and the ONLY TRUE 4K camera on the market - the Sony F65. That camera is actually 8k ready. Just wait. It's still in its infancy shooting movies like Oblivion and After Earth (terrible). But check this out https://vimeo.com/81600295
(I can tell you right now if Belle isn't up for next year's Oscars they're crazy, not just for the cinematography, but the amazing story).
5. Canon's low light performance is really well implemented in all three cinema EOS cameras (No I'm not including the 1DC because I'm not a fan of the video on it nor its highly compressed 4k, but it's decent, not worth $12k though).
There are films using the C500 as B cams, but as A cams you do have short features such as this lovely one shot by Edoardo Ponti(Son of Sofia Loren) https://vimeo.com/81420176
(Human Voice) shot in 2k because they would rather have 12 bit 2k flexibility in post than 10 bit 4k (not work $22,000).
Anyway I'm a HUGE fan of Larry Thorpe, I think that as Canon enters the Cinema realm they've only just begun so it'll be interesting to see what lovely cameras are coming in the future.