Are they only effective against air and workers? They don't seems to be powerfull against troops mechs and hvy weaps...:(

Well I know when I play against the Gungans I use them against those annoyng Fambas.

No air unit is that powerful, basically; You'll just have to group them and use them for hit-and-fade missions to get results.

Well, they'll do most troopers in a single hit. But their slow fire rate doesn't do them any favours.

Then again, Jedi Starfighters are not primarily combat craft; that's why you've got the much cheaper fighters. Starfighters are better used in small numbers for TL4 recon...

I use them for spying and worker killing (but there is not much else you can do)

well if your opponent has air superiority and they are using air cruisers on you, you can use them to sneak up on the AC's and destroy them. The delay is short enough that unless they have A LOT of fighters right next to the starfighters, there is nothing they can do to save the air cruiser by the time the starfighters uncloak.

and T4 worker killing is the BEST use for them. You can keep that up until your opponent puts sentry posts everywhere (which most people wont until its too late)