I briefly investigated several similar-purposed gems including: PaperTrail, Vestal versions, audited, Acts as versioned, Userstamp , and found that:
PaperTrail, Vestal versions and Acts as versioned are all very powerful versioning gem. Yes, you can also use it to audit/track users' activities , but it's not very straight forward because they are 'versioning tools', but not for the 'auditing purpose'
Userstamp and audited are for auditing purposing only. but Userstamp is for older Rails versions ( it's a plugin... which is not supported by Rails3(or 3.1, 3.2?) anymore )
so I think 'audited' is a better choice for auditing user activities purpose (not considering model versioning) for Rails3, this is enough for me. Further more, it supports associations and ActiveRecord/MongoMapper.
I hope this helps.
