Yes they work just fine, in fact that is how I had my system running until I got my opensprinkler pi in the mail. The only problem that I really had with it was that it is normally high so when the raspberry pi goes down or something, your sprinklers come on. Doh! So I would recommend you get the solid state one that is normally low from here – http://www.sainsmart.com/8-channel-5v-solid-state-relay-module-board-omron-ssr-4-pic-arm-avr-dsp-arduino.html
Also you are going to need a shift register because the program isn’t written to use a pin per relay, so you can get those here
All in all, honestly I would just spend the extra $25 to get a opensprinkler pi. I tried going the relay route and spent more $ and time than I would have on the opensprinkler pi (and I still ended up buying it).