Excellent pump, working just fine for 10+ brews without any drama. Easy to clean, easy to use, and just works well. (I now actually have 2)
If you are not a tighten until it breaks type person, you can get away with the plastic head, it's strong and durable. BUT, I did opt for the stainless head (I do go back and forth depending on if I pulled the valve or not, and do I feel like removing the teflon tape at that moment) but now I feel it was not necessary.
And Williamsbrewing customer service is Excellent.
Another recommendation for the MarkII. Quiet, works great, and I bought the stainless head just to be safe. Never even tried the poly one. Several places carry it now. I recently saw Morebeer started carrying them.