I have the one cd at a time Netflix plan with on-line movie access, the $8 plan.
I have The slower DSL service in my area through our phone company.
The DVD service is pretty good. Usually we have a new DVD on the second day after mailing the watched DVD, so about 48 hours turn around time from DVD out to DVD in. Sometimes it will come on the third day, but not often. I wouldn't call us heavy users for the DVDs.
As far as the on-line service since my girlfriend and I both use it together and independently so I would say we are heavy users. We do get the adjusting speed thing regularly once maybe twice per movie. I have gotten the movie ending type of error once, but it was during peak internet usage times. We eat breakfast/dinner in that block of time on a regular basis so we don't see it very often. I would say we are heavy on-line users.
Chances are if you want to watch a movie just as people are getting to or home from work/school you are in for a somewhat disappointing experience on a regular high-speed connection. But, if you avoid watching during the busy internet times you will be very happy with your $8 a month service. How much of the bandwidth issue is attributable to Netflix from my perspective is a little murky to speculate on since I have zero facts besides my own experience.