I'm really happy with my G2x so far. I picked it up on Friday afternoon, and have not had any issues so far. The batter was completely dead when I got it, so be prepared to wait a bit before you start playing with your new toy.
Build Quality: The phone is heavy, but not a burden. You will know it is there, but it won't be pulling your pants or purse down. The glass feels great, and the seal between all the seams is perfect. The back is easy to get off, but also very secure (luckily, I haven't done a drop test yet). The buttons are very firmly seated and have a clear 'click' when they are pressed, which is really nice.
Perceived Visuals: Quadrant Benchmark has topped at around 2450 on Friday night. Yesterday and today it is at around 2325. Games run absolutely fluidly (both pre-installed and downloaded), and the screen looks great. I know that many will wait to pick up the HTC Sensation, but the screen on this is more than adequate unless you are REALLY nit-picky (I would categorize myself as moderately nit-picky as far as this goes!).
I have not played with the HDMI mirroring yet, so I will have to assume that it does what it is supposed to.
Touch Screen: As noted above, there have been a few times when the screen is not responsive to my touch, but I think this has always been in an app, and so I am inclined to blame the app for bogging down for a sec more than anything else. Other than that, I find the screen very responsive. The glass has a great feel to it, and typing in landscape is as simple as it can be. The keys are well spaced and the response to typing is just as it should be.
Connection/Voice: The 4g speeds work really well (I'm lucky enough to live where I can take advantage of that). Most apps download in about ten seconds. Text messages send in about 1.5, and email is fluid. I have gmail, IMAP through the email program, and the Yahoo mail app all running, and they work great. I was having a lot of trouble with the new Amazon Cloud Drive playing my music, but I switched over to WiFi and the issues continued. I then switched to Pandora and there were no issues, so I think the problem is the Amazon app (among its many issues - but I digress). Voice connection is very clear, and I have no issues with volume levels. I have not tried the speakerphone yet, but the music from the speakers is loud and clear, so I image the speakerphone will follow suit.
Ergonomics: This of course has to do with me, my past phone experience, and my usage, so you can take it all with a grain of salt if you choose. I find the lack of any physical key an issue because unlocking a phone with just one unlock key on the top makes the act a two-hand process, especially since the unlock button is on the right side and I am right handed. I have to hold the phone with one hand and hit the button with the other. (I can do it one-handed easily with my left hand, but as I am right-handed this is not my natural predilection.) This also has to do with the size of the phone, which is quite a bit bigger than my last. That being said, all the capacitive buttons are clearly separated and easy to reach with one hand, so one-hand use is still possible. Next, I find the top edge of the phone where the speaker is very sharp, which, when pressing the earpiece against my ear, can become uncomfortable. I don't need to press it since the sound is plenty loud, but old habits die hard, and I find myself having to remember to pull it away or trying to reposition it a bit. It would have been nice to see the top of the phone rounded as beautifully as the sides!
Software: For me, Vanilla Android was one of the biggest sellers on this phone. I've been running a rooted and 2.2 rom'ed CliqXT for a while, and seeing how a non-skinned version of Android could run on even sub-par hardware really made me glad that this phone came out without any LG overlays. Again, I know that many will wait for the Sensation and will be very happy with Sense 3, but for me, Vanilla is good. I can put flare and function on my home screen with Fancy Widget and I customize my unlock screen with Widget Locker, and after that I am good to go! T-Mobile does pre-install a bunch of bloatware that is CONSTANTLY open, so I am really excited for someone to post root procedures for this phone. One great example is two different programs to help me find apps - as if the internet, app store (Google or Amazon), and Tegra Zone weren't enough. Programs like that just don't make sense.
Battery Life: What can I say? I run the phone hard and the battery dies. I turn stuff off, and it lasts longer. I would say that MY average usage (constant email checking by the phone, sending some texts, a few twitter posts, 30 min on the phone w/o bluetooth, a few minutes of gaming, and looking up info for a movie and dinner date) shouldn't run you dry in less than a day. Streaming from Amazon Cloud or sitting playing Dungeon Defenders for a hour, however, is probably going to have you reaching for the power cord by dinner time or earlier (esp. for the latter).
Overall, I am happy with my first three days with the phone. If you are up for a full upgrade, I don't think you can go wrong unless the Sensation's upped ram and resolution is to good to pass up. I will post a reply/edit if something major happens, but I think that this phone is well made and will last. I had VERY high hopes for this phone, and I have not been disappointed with my brief time with it.