It should work fine with anonymous delegates, e.g.

var fives = yourEnumerator.Where(delegate(int x) { return x == 5; });

With some extra testing, basic lambdas work

var fives = yourEnumerator.Where(x=>x==5);

There are some still a few issues, but you can code around those most likely

Older question but I thought I'd add my experience.

I work on a unity 3.5.X project that targets desktop and Android (specifically touchscreen PCs and Android tablets). We use Linq extensively in out project and have not run into any issues yet.

That being said the client side queries are quite simple (where, min, max, orderby, select). We do use a lot of the extension methods as well (ToList, SelectAtIndex etc). Haven't had a problem yet (other than MonoDevelop completion not being up to par with VisualStudio).

Haven't targeted iOS, so I can't speak to that.