I like doing business with Quest--the software is dependable and solid, their support is accurate, responsive and the followup excellent. Their a good company to do business with (plus we also use TOAD)
There isn't much to dislike, other than a bug in the version 5.3 of the software causing our older attachments to not show properly. However, once patched, everything was back to normal.
Don't skimp on your hardware--give it the horsepower to do the job you're asking it to do! We're running it on a dual-processor 2U server with 12TB of internal storage and 32GB of RAM--and it will use it! We like it on a physical box in lieu of virtual mostly because of maintenance and the ability to keep it separate from our Exchange server. The only thing I'd like to see is maybe a more robust relationship with a DB vendor.
We use Archive Manager to archive our Exchange server and attachments. Because we're a public institution, we have long retention times for our email archives--and Quest Archive Manager has done a great job fulfilling those requirements.