I would not recommend SpamAssassin. SA is a great product but it is slower than other open source products and requires substantialy more maintenance (updating rules, etc) then other open source products.

If you are looking for a great overall product with good interface to manage users and a good interface for the users managing their spam/ham then I would recommend DSPAM. It has many different options and allows you to choose between many engines for classifying spam/ham and many different operation modes. To my knowledge it is the only pure statistical solution having support for classification groups, merged groups, etc. This allows you to pre-train the filter and offer out of the box high capture rate for new users while still allowing the user to tweak/train the filter for his own needs.

If you are looking for fast processing and high capture rate then look at CRM114 or OSBF-LUA.

OSBF-LUA is one of the fastest classifier I have used. It very fast and does not need much training. Beside that it is pure statistical (like CRM114 and DSPAM) and does not use any external sources like RBLS, RHBL, DNSBL, DNSWL, reputation services, SPF, DKIM, hashing, etc (DSPAM can use external sources but does not need to in order to have high catch rate).

OSBF-LUA is the winner of TREC's Spam Track 2006. It outperformed any competitor including SpamAssassin and commercial products. I think this says enough about the quality of the tool.