Might I suggest that if you find Windows Server Backup to be unreliable, you should look into the root cause of that before you look to get new software. You may have problems that will follow you no matter which backup software you use.
I've found Windows Server Backup to be reliable and have performed a full image restoration with success. The only trouble comes when you have separate applications that need special agents, such as databases. Your experiences of unreliability should be broken down to an atomic level to discern what exactly the problem is.
FYI, your post is what is referred to as a "shopping question" and will likely be closed shortly as per the ServerFault FAQ. However, consider Symantec Backup Exec, AppAssure Replay or FalconStor DiskSafe. No links are provided so that no implication of endorsement exists.