Yeah those time signatures are weird at first. What I usually do is write the drum part first, then play something over top of it. Those odd signatures like 5/4, 7/8, and 9/8 become alittle easier if you have something like drums already playing the feel. Then once I got used to this method, writting in those time signatures became alot easier. If you have a drum machine or software (I generally use fruityloops), you can write up a drum pattern and convert it to wav (mp3)file and just jam over it.

Take a 5/4 drum beat.
h:--x---x---x---x---x---
s:----------x-------x---
b:--x---x-------x-x-----

Each hi-hat hit is one beat. It's a really simple beat, but you can tell it's 5/4 very easily. And it's probably just as easy to put something to it. 5/4 is made up of two signatures, 3/4 and 2/4. This gives this time signature a sense of incompleteness.

Also check out bands like Az said, like meshuggah and also quite a few 70's classic rock bands use odd time signatures.