We also run Exchange 5.5, and found it running rather slow (occasional 30 sec delays, etc). We looked at GFI but did not go that route because our server may be underpowered.
What we did go with was a Barracuda 300. The 300 has one nice advantage over the next model down; it supports LDAP, which stops any mail that is going to an invalid email adr. This means the server never sees it and never has to waste CPU.
The machine gets mail and then sends it to the Exchange server, so the load on the server is much less. Our spam runs about 95% and we were at that level with a little tuning (their user interface is great, very easy to learn!). Their support is very good on big problems, but sluggish on minor things. They have a very active user forum (sponsered by Barracuda Networks), and is extremely helpful.
I probably shaved another 1% off of spam getting through by tweaking other things and doing some keyword filtering, etc, but I think I just caught stuff until the next signature update came out from the Barracuda subscription (the sig can be set to update every hour and works great!)
The slowdowns we see in server response are now mostly gone, the remaining probably due to large file attachments.
gr
