I think Groupon suits the purpose if you would be visiting a restaurant that has a usually high amount of coupons or vouchers changing hands. Here in the UK if Franky & Bennys offered the coupon I don't think I would think twice. I think also if it was for somewhere I frequented often and I know the standard of food is good then I would do also.

We had a situation earlier this year with a voucher where the terms were not fully explained, we went to Manchester (England) and sat down in a restaurant called Giraffes. The voucher did not explain that it could not be used before 7pm. It was 1 in the afternoon, I sat there regardless and continued to be ripped out (£8.50 for a breakfast, cheapest thing on menu) and in total paid £40 for some dinner.

In terms of tipping, I generally leave a tip if around 10%, more if the person waiting is really good and a good personality, less if I think they are over acting it to get a higher tip. They don't get anything if the food is crap or over priced. I don't think I have been anywhere recently where I have not used a voucher or coupon, it's not because I am tight, it's simply because they are there. I don't consider myself a bad tipper either.

I know what you mean though about feeling bad about using vouchers or coupons, I feel slightly guilty about it. I think the English culture is slightly different though, I think we don't like confrontation and I feel this goes into that realm.

Stick to voucher cloud, check the terms though properly before sitting down!