it uses the same base code as the bitcoin client, so it's just as secure as bitcoin. however, any keys created prior to the wallet's encryption may still be vulnerable.
Also note that your wallet comes with about 100 addresses preloaded, just not visible (
https://en.bitcoin.it/wiki/Securing_your_wallet). This means, there are more keys in your wallet than you can see.
Key creation might not happen when you would think it happens. Take this into consideration as well...
Hope this helps.
T