To be pedantic, you don't need a FI to instruct ME/IR ... as long as you are a CRI/IRI on the appropriate class. In fact, the guy I mentioned earlier did not have his FI by the time we were flying. Long time since I read the JAR-FCL, but I believe you can do it since you are not "teaching towards a certificate".
To change side here...
I work as CFII/MEI on a more westerly longitude now... and I agree with protectthehornet, it's fun (of course) but I cannot see myself instruct everyday for the rest of my life. That would kill me (mentally... and possibly figuratively). I try to combine flying as much as possible with other things, e.g. teaching ground school, administrative matters, whatever I can find. It's the change that keeps me going!!!