To add:
Like Phil wrote: Cinestyle (or any other flat-style) is extremely valuable if you really want to get the last bit of quality out of your footage in
post. So you'd have to put a lot of work into grading.
There are two excellent articles by Stu Maschwitz on his Prolost blog about shooting flat in general. In the second one he also mentions (and recommends) the Technicolor Cinestyle.
You might want to check them out:
http://prolost.com/blog/2009/8/3/flatten-your-5d.html
http://prolost.com/blog/2012/4/10/prolost-flat.html
I personally like to use the Cinestyle for my own projects, but I'd be also very careful if you have to hand the footage over to another editor or if you're showing it to your client. The "RAW"-footage looks kind of awful ;)
I did a couple of shoots lately with a picture style called "Kodachrome 25" which is
not a flat-style, but tries to emulate the look of Kodak-films. This is something I use, if I want to have really nice and "warm"-looking footage out-of-camera without having to grade the heck out of my clips to look good... Downside is, you don't have too much room left for any kind of tweaking in post...
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