Check out all the cool games that are being built with the Unreal engine, and talk to the developers to get a reference. Read what the reviewers said. Check out all the mods enthusiasts are developing with the publicly available UT stuff. Look at the worldwide sales charts and the track record of the Unreal games. Repeat this process for all the engines you're considering, and draw your own conclusions.
Legend is a great partner.
They're one of the few companies where artistic people and technical people work together in harmony. They're well into Unreal 2 development, using a combination of our latest engine features and their own. Engines are driven by the same critical mass and network economic effects that drive most prominent products, so what you tend to see is a lot of competitors vying to get in, but only two or three companies really succeeding. Others exist, but most people don't even know they exist, due to such low market share. It's interesting how that pattern, of 2 or 3 competitors taking home nearly all the business, applies to so many markets.