Value for Money
I have been using Discovery Attendar for 4-5 years now. Its fairly easy to use and light weight enough to run on a desktop and search 50,000 + emails. Works very will with email stored in .pst files. Other email file types may need help from tech support but they have been responsive in writing patches when needed.
Ease of use.
Can't open every email, but can over 99% of them.