Assuming a full-tunnel configuration, your data is protected between your computer and the VPN service, and from a web site's perspective your traffic will appear to originate with the VPN provider.
However, you are now placing a lot of trust in the VPN service. They do have un-encrypted access to all of your traffic. Additionally, anyone upstream to the internet from the VPN service will have access to your traffic (ie, this won't help you hide from the NSA).
A trusted, reputable VPN service is useful for places like coffee shops, where you're stuck with un-encrypted wifi and anyone else in the shop could trivially listen to your traffic. Here, the VPN would encrypt that traffic while in transit over the wifi network, all the way to VPN provider.
I also need to mention that OpenVPN also supports split-tunnel connections. A split-tunnel connection means the VPN link is only used to access resources on the VPN network itself. Split tunnel is useful for things like access to resources at your workplace, without having to force all of your regular internet browsing traffic to run through the workplace WAN connection twice.