However, I'd suggest a better question is whether it's secure *enough* for your purposes. After all, not much (if anything) is 100% secure. Do you need military grade security for transferring state secrets? Or are you just sending the details of your shopping list.
I guess if I was you I'd have a think about what level of security is appropriate for my needs, then have a word with the pushbullet support folks at the link above.
Read the privacy policy - you should judge for yourself. I did, and so did my IT rep. We both agreed that Pushbullet is a security risk. I liked the promise of convenience, but it's not worth the risk. IMHO, it's another marketing whore.