I don't really have much experience of wearing them for any length of time, but like you I'd like to know if they are worthwhile and the best type/brand to get. I have visions of them simply pushing my flab somewhere else though
I did once buy a pair of 'control' knickers from M&S but they gave me tummy ache!!! Likewise when I was going to a wedding I tried on one of those M&S body slimmer things (like a swimming costume), I felt like I was suffocating but it did make me look a lot slimmer. I have to admit that I didn't buy it though because I find being able to breathe/bend/sit etc is quite handy!!
I guess it probably depends on the strength of control and the area being controlled, and possibly your own level of comfort/discomfort. I suppose they could quite well feel tight because if they didn't then they wouldn't be doing their job, then again I may have been trying on a size too small!!!