Is a keg too big of a mash tun for smaller batches?

I've been putting alot of thought into starting to go all grain. Right now I only have a 8 gallon brew pot and I since I purchased it fairly recently I know the wife would have my head if I got rid of it so soon. My plan is to convert a keg into a mash tun since kegs can be purchased fairly cheap and I can do the work myself. Is the keg going to have any downsides since i'm only brewing 5 or 6 gallon batches?

Thanks

If you batch sparge it will work fine, though a cooler and braid is probably a better solution if you don't care about direct fired step mashes.

none at all.. i do both 5 and 10 gal batches.. just depends on the beer..

Thanks for the info!