Nothing beats a quick look at the pressure charts, tide tables coupled with knowledge of your local breaks for getting predicting surf.
The biggest problems with windguru and magicseaweed which use very simillar if not the same computer forecast models is that whenever strong winds are forecast then it automatically puts in a high wave size.
Both guru and seaweed regulary forecast waves of 20-30ft for my local breaks Which of course is absolute nonsense.
Waves of 12-15ft are forecast for bournemouth pier tomorrow whien in reality there will be 2-3ft of wind chop, more if im lucky as i feel a bit of cold coming on and may have to take the day off!
KS