Tools, Sharpening and Power Machinery - Chainsaw Sharpening?

On average I'll cut about 2 cords of wood a year. I've been sharpening my own chains with a round file. I'm probably not very good at it, but it seems to work. I've been looking at some (cheap) sharpeners at the hardware store.

Some that I have looked at are:
-the kind that clamp on to the bar (around $18) and hand file
-the bit that you can put in a drill ($8) which has a cheap guide of sorts

I'm looking for opinions on good or bad experiences with either of these sharpeners... what's your experience?

Thanks for your help!!

I use one of the bits and guides in a roto zip to keep mine sharp for short term. I don't think, without alot of experience, you and I will ever get the sharpness that a professional sharpener will give. Not only do you have to sharpen the tooth, but you have to adjust the height of the follower as well, and I wind up sending mine off after 3 or 4 of my sharpenings.

I've used both the file and a stone mounted on my dremel. I think the file does a better job but the dremel is easier:D
I've never used a guide but seem to do fairly well by eyeball. I usually have the chain sharpened professionally about half way thru the chains life. As Chandler said, they do more than just sharpen the teeth.