I had occasion to use MWB yesterday and picked up the latest free one, skipping the "free trial" option on the install.
It found a goodly number of "nasties" on the drive I was scanning (from another system, connected externally) but I had reason to suspect not all.
A follow-up scan with Trend Housecall found a couple more fairly major items.
It did make me wonder whether the differentiation between free and paid is maybe a bit too much, with the free version missing some fairly obvious things and pro going OTT.
I do tend to use MWB as my first tool to attack Malware, but normally use at least 3 different scanners in total before I'm happy it's clean.