I'm researching the effectiveness as well. Here's an article that was worth reading - as in anything else though, I'm sure there are a variety of opinions and since I'm new to this too...hopefully those with more experience will enlighten us
http://searchenginewatch.com/showPage.html?page=2167931
I think by default the system will plug in the product name as is. I'm not sure how well worth while in terms of this area helping you with the search engines - but there seems to be a few more characters to play with in the meta title.
The Description area of the Meta Tag is somewhat important (except for Google - they ignore this and take their information from the description area on the page) - but others do support it and it's suppose to give you some level of control with the crawlers. I think the only thing the system pulls for the meta on this one is your Category & Sub Category names - I'm not sure if it pulls anything from the description area (maybe someone else knows for sure). You certainly have quite a few characters to expand your description for those crawlers that still pick it up though.
As for your keyword choices - I read in a couple articles, that if you have keywords that might be searched on in different ways, to make sure you are using both ways in your actual description to capture as much as you can (the example was Horseback Riding vs. Horse Back Riding) - you may want to include both in the keywords too. But from what I can gather, most major crawlers are now ignoring this tag completely but for those that still do, it may be worthwhile to plug in - here's another page worth checking out (it shows how a lot of the major search engines work and what they support)
http://searchenginewatch.com/showPage.html?page=2167891
Also if it helps...I just did a search yesterday on "50th Wedding Anniversary Invitations" LOL!
But I'd love to hear some opinions from our fellow 3D'ers to see what they've done. I'm probably going to quickly set up one in each of my categories, then do a lot of cut & pasting in the product option template/csv file to import back in. However, I'm still not 100% sold/convinced that it's even worth my time & effort to work on the Meta Tags with the more articles I read...
More thoughts anyone? Suggestions? Tips?