The GT25 is very much an incremental update to their G25 line of TVs, so the design, remote, port selection, and general user experience are pretty much the same. The initial setup wizard lets you choose the language, switch between home mode and store demo mode, configure your various video sources (cable, antenna, etc.) and set up your Internet connection (via Ethernet or USB Wi-Fi dongle, sold separately).
The included Viera Cast features include Netflix, Amazon Video on Demand, and YouTube, as well as Internet apps like Skype and Twitter. There's also a neat Fox Sports app that gives you readily-accessible sports scores and news. Configuring them was pretty easy, although typing with the remote is certainly a chore (imagine texting with a number pad on your cell phone, but clunkier). Note that you can use either the USB ports or the SD Card slot to view movies and pictures.
If you don't feel like calibrating your TV, the GT25 has a THX-certified preset that comes in handy. The THX mode is the easiest way to get the set looking good, though you should keep in mind that it was designed for a rather dimly-lit room (with pitch-dark room being ideal). There's also an included Game Mode, which turns the contrast up a bit too much for my liking, but claims to reduce response times (most likely by disabling certain image-processing features) so you won't have too much lag when you play.
Rather than rehash all features from the G25 line, I'll focus on the two big differences: Image quality and 3D features.
Our jury came down on the G25 for oversaturated colors and overly dark blacks--while the blacks were indeed very deep, they seemed a little bit too deep, as though the TV was "rounding down" the shades of dark gray into black. The net effect was that the colors were certainly vivid, but the oversaturation made everyone in our test clips look a bit sun-burnt, and the too-dark blacks meant we lost a lot of detail in dimly-lit scenes.