My husband and I were on the DDP during our Feb trip and all of the TS restaurants we dined at were very accommodating and friendly.
It was our first DW trip so I had never used a dining plan, but I did a lot of research beforehand here on Disboards about how the credits worked so we didn't need a lot of guidance from the waiters/waitresses. The only question I really needed to ask was which beverages at the specific restaurants we dined at were included in our plan. Each time I asked, the staff was incredibly friendly and helpful.
Usually they asked us if we were on a dining plan right when we were seated and peeked at our card, assumingly so they could guide us through the ordering process if we needed help. Not once did I feel looked down on or like I was hassling them or an inconvenience to them. Every one treated us very well!
As someone else mentioned, I'm sure there are folks who do not leave a tip while on a dining plan. I know tips used to be included, but people should read the brochure so they aren't leaving the servers out of a part of their wage. The dining bill makes it pretty clear where to write in a tip amount, and I could be wrong, but I thought somewhere on the bill it said that tips were not included or something like that. I do remember a couple of our servers casually (not in an unkind way) mention to us how to properly fill out the tip section. We weren't offended by that.