What I've noticed is that if you pay for premium service, the apps keep the connection up way better and offer nice features, like switching your country from an easy menu.

Tha being said I've been quite happy with http://privateinternetaccess.com/. A good Android client, fast, don't ask any personal information when you register, they say no logging, the Android app doesn't ask any permissions and so on. I can choose the key length and additionally block all connections if the VPN is not connected.