For us this has already been decided - Enterprise Vault.
Yep, after years of having no e-mail archiving solution and relying on PSTs combined with a manual archiving process for terabytes of e-mails we're not jumping in whole hog!
Fingers crossed the contract will be signed this week and then we can start really getting our geek on planning the roll out of this so we can get to Exchange 2010. Not bad considering I've still got an Exchange 2000 Enterprise server kicking about somewhere. :)
We are currently using Enterprise Vault, and it is pretty good for Email Archival. The biggest issue we have had is importing PST's that are currupt (Go figure, that never happens...), and users losing archived email.
Making sure that the Outlook plugin is started is also another big issue, because the user can accidentally turn it off.