We had a blast going to Oktoberfest for (parts of) 2 days. My wife and I went on the 2nd Monday and Tuesday (9/26-27 this year) and didn't have any reservations at all. We hit 2 different tents each day and had no problems finding spots at a table at each tent (both before or after 5pm). Met a lot of great people, all of whom were locals, or at least German. Liters were 10.xx Euros each, so about $6/pint. Expensive for Germany/Europe, but no more than I am used to paying in Chicago.
I'm in no rush to go back, as I have a ton of other places on my list, but would definitely attend again. And I wouldn't worry about reservations as long as you aren't part of a big group. Though, I was told by many people that attendance has been down each of the last 2 years with the terrorism that has been taking place in Europe. Security seemed pretty strong without being overbearing entering the Wiesn.
I would definitely recommend going on a weekday. The energy was high without it being a total ********.