Intermitent data streams, drop outs, lower transfer rate, higher chance of neighbour's trying to hack/steal your connection...
pro's.. it's wireless, you can use it all around the house (most places) you're not tied down by a cable or restricted to one room..
personally I have both, I have a 8port wireless hub, I run 3 desktop's hardwired, ps3 and wii hardwired, 2 laptops wireless and ipad/ipods wireless.
Best of both worlds for me, also use a decent WEP key so your neighbour's don't leech off ur internet.