I know it's a stupid question, but they'll stop the bike a bit…won't they?

I have had commuter problems today, and am nearing the end of my tether.

Your brakes will work, at least sort of, but may wear through the rim more quickly than a std rim. (i.e. prob OK short term in dry)

Cheers Al, they are spare wheels (cheap and old) so at least it will get me to work in the short term.

You may need to adjust the angle of the brake pads as some rims are not "aligned" the same way as a braking surface. In extremis, you may find that you do not get full pad contact on the rim. This will obviously accelerate wear where the rim and pad are in contact.