I've currently got a dremel but it gets REALLY hot after an hour or so of constant use so I'm looking to buy an air powered mini die grinder (the pencil type) but not sure which one to get - so, does anyone here use one? They seem to range from £20 up to £100+ but the info on each one is very vague.

I've got one you can have. Its a cheapo one.  The downside with the cheap ones (whatever brand they are) is that the chuck isn't bored dead centre.  Fine for a cutting wheel but not so good for porting cylinder heads with a carbide burr.  If you want it PM me your address.

I have this one for little bits

When they first came out you could rely on the quality now is a different matter

Dosent matter what anyone tells you the quality in the £15 isn't that good but its ok

All depends what you want from it really

I have a Rodac- £150 ish I seem to remember (top end stuff from back in the day and will do a true 100,000 rpm smooth as silk) and a Bergen-£45 (similar to the Sealey )

Cant fault the Bergen and its held up through 2 busy F1 car builds so far but its just not as good as the Rodac-power/feel/response

Work has just purchased 4 from Chicago pneumatic (about£75 each)-they seem very good