Can I ask regardless of what backup solution you use:
1) Have you ever had backup jobs fail? If so what caused them to fail, and is it quite common for them to fail? Or a once a year type scenario?
2) What mechanisms do you have in place to identify a backup job that didnt complete successfully, and what can you do about a failed backup job? i.e. if it failed it failed, theres no much you can do about it?
Re files being locked, can you provide an example? I.e. users with files open? Or something else (please keep answers management / low tech freindly)...