use commercial spam filtering service?

[Jeremy C. Reed]

Some of my FreeBSD and Linux customers have own mail servers, but use 
third party companies to be the MX for their domains.

Do anyone of you use a commercial spam filtering service?

Please share your thoughts about it.

Do you use it to supplement other spam filtering?

Do you just use it as the main MX and it then relays legitimate email on 
to your real mail server?

How did you decide it is worth its value?

Do you have any research to show how beneficial it is for you?

Any significant lost mail or significant delays or other problems?

Do you also pay to have your outbound mail relayed through their gateways 
too?

As you can see I am researching this :) If you know of any good webpages 
or news articles that compare different companies, please let me know. 
(This seems like something that InfoWorld reviewed ... I am searching ...) 

  Jeremy C. Reed