Always open to new suggestions, fo3nix, I decided to try Thunderbird the last time you suggested it. So I've been running it for the last two weeks.
The biggest disadvantage is that TB downloads all the e-mails including their attachments before deciding whether or not they're junk.
All the time I was running Mailwasher, my incoming virus checker was largely redundant, because viral attachments didn't even get downloaded.
Also with MW I was getting around 10% false negatives and less than 1% false positives, whereas with TB alone it was more like 25% false negatives and 30% false positives.
MW uses a combination of bayesian, lookups, manually entered rules, whitelist, blacklist. It also allows you to use wildcard entries in the white- and black-lists.
After my trial , I've decided to stick with TB as my client, but keep MW as my spam blocker.