They are definitely trustable - many people use/trust them. Is that a good idea? I know they have taken steps to improve security. They have not been hacked again, but that doesn't mean it won't happen. They are obviously a valuable target, but so is any exchange that has any volume.
I think what is more important is weather you trust them to reimburse you if someone does hack their site. Any site can be hacked and there is no real way to tell how good their security is without being a security expert yourself. This goes for any online business that deals with other people's money or financial information.
The best thing to do is only deal with companies that have good reputations (search for them on this forum or google) and don't ever keep large amounts of cash in someone else's pocket longer than you have to. If you have $100 (for example) to convert to Bitcoins, the best strategy is to break it into several smaller chunks and convert them one at a time or send each to a different exchange. This will decrease your risk should one of those sites get hacked. If you do the exchanges over a period of time, you will also reduce your currency risk (remember, Bitcoin can be volatile) a little bit by spreading it over several transactions.