Is iRule case sensitive?

Yes, very. If you're looking to compare strings you could always use the string command with the tolower function. It forces the string to be evaluated as if it consists of lowercase characters only. An example:

[string tolower [HTTP::uri]]

This will return the URI converted to all lowercases.

the string compare is one thing, which in my opinion is almost mandatory for any good script / programming language.

but also the commands and such are case sensitive.

something like: SET var [http::HOST] won't work, it has to be set var [HTTP::host]