We run a relatively small non-profit site with 500k posts, 20k members, and 1-2 million pageviews/month. We've spent nearly 12 months planning our migration from vBulletin to xenForo. After getting everything configured out of box, we had 23 custom add-on requests, all of which ForumCube completed. Here is my review of their services:
Price: 10/10
Their quotes were fair and beat the competition by a long shot. But the main thing we really appreciated was how they stuck to their quotes. Sometimes there were changes to requirements and effort, but they always stuck to the quote. This was incredibly professional and we trusted them very much because of that.
Quality: 10/10
All of the add-ons do exactly what we asked for. I write requirements for a living, so I'm sure I was a bit perfectionist/annoying to deal with. Still, they took our mockups and written requirements, turning it all into a reality exactly as we imagined. There were minimal queries, and no performance hits whatsoever.
Speed: 10/10
Well, ForumCube would probably give me a 1/10

I couldn't keep up with them! I'd take a month or two to write the requirements (38 pages long), send them along, and within weeks they were ready to test! All of my questions and suggestions were promptly implemented in their QA environment by the next day.
Overall Experience: 10/10
Obviously 10/10 given the above. These guys are fast, professional, and great at what they do. They offered free support for our add-ons when we go live, which is such a huge relief. They also included some free improvements to our requests, which I know required more effort and complexity. Not only do they implement your requirements, they use their technical expertise to propose even better solutions. We can't wait to work with them again in the future, as I'm sure we'll have a bajillion more requests over the coming months.