I think the mugs are wonderful! I love the colors and the design of the mug. Although the image is really small (I was hoping you could at least read it), I still think they look great. I think Angela had a valid point in noting that your guest would be more apt to continue using them after the wedding with a smaller logo.
However, with all that aside, I think it's really pathetic that the company sent you the mugs without your approval. Is it their standard practice to send you a proof first? If so, I would demand some type of retribution- especially since you are not happy with them. That's what's important- your happiness.
I don't know about everyone else, I love this forum for so many reasons. One of them is the ability to reach hundreds of brides to notify them of crappy service and warn them about the potential mishaps should they do business with the company. It's sweet revenge for poor customer service. I hope this company makes the effort to make it right.