SpamTitan offers some of the best filtering we've seen compared to other products. It does an excellent job when configured right, of capturing a high volume of spam. It's relatively simple to get around and set up and runs in a very lightweight VMware appliance.
For us personally as well the price point is quite cost effective vs. other products on the market, and they are flexible in the pricing models offered.
We use Spamtitan to add another layer of protection against all potential outside attackers of spam, phishing, malware, and viruses. With SpamTitan we keep our users far safer than with stock filtering of Office 365 or anything else I've used over my 10+ year career (and I've sampled quite a few products).
We see a daily significant reduction of spam, malware and other attacks against our users from the email front - and for the small percentage of stuff that does get through - we have other layers to ensure the end user is as protected as possible.