We are using 7.2 IOS and it works fine.
Performance wise it is ok.
But when it comes to reliability you have to work on the following:
- Firewalls in Failover (HA), so that even if the primary FW fails, the vpn tunnel works via the backup FW.
- Have a backup internet link, so that if primary link goes down the tunnel on the backup link works.
Yes its (usaully) reliable, just make sure you upgrade to the latest minor release in your train like 7.2(4) etc.