You may want to spend some time in the sections that concern you but don't get caught up in the mentality that everyone has has a problem with "xyz" issue since in most cases people with problems are very vocal and complain looking for a solution. You have to put things in perspective and realize that most owners don't belong to a website, then majority don't post things like "my vehicle did great today and I didn't have any problems".
To get a more fair and balanced idea of AV ownership read threads about high mileage or multiple owner AVs. Then each year we get optimist who think this year is the year that GM is going to reintroduce the next gen AV etc.
As far as reliability I haven't read of bad things about the 6 speed, just people getting used to the difference in feel and some lack of towing ability. Then as far as the 4 speed I think that all depended on the owner and their needs or driving style, some members have been through several transmissions and considered it at it's design limits and when they rebuilt it they put in heavier internals. Many members have done well on the 4 speed tranny and have not had issues.
As far as specific used AVs you need to look at it's maintenance history and exposure to extreme conditions to decide if that vehicle is a good buy or not.
A sight like www.kbb.com or www.edmunds.com can help you determine a good buy in your area, just remember that things like a CarFax report is only as good as info provided, if an owner or small repair shop failed to report an accident or other problem it may not show up.