I have Object Desktop, and until the yearly subscription runs out I am good to download any upgrades (meaning any upgrades that come out and are available during the year of my subscription and are included with Object Desktop, which Windowblinds is) unless I don't have to reload after that sub is up (if you have to reinstall it and your yearly sub is up, now Impulse doesn't let you download the products unless you renew - or at least it didn't let me, but I didn't know my sub was expired until then)...
Object Desktop is totally worth it - I have renewed my sub for the past 3 or 4 years, which is saying a lot for me (to stick with something that long). 
I've never bought the products individually because it was too expensive to do that. LOL Not sure how it works for an individual product...but seems like from others' comments you'd be good to go...unless it works like Object Desktop and you have to reinstall after your sub is up, in which case you couldn't download any updates or new products until you renewed your sub, which is basically what Anakey said - so I'd def go with Object Desktop! LOL 