Spent much of the last two days playing around with the 55" LA7400, side by side with a Sony 50" R550A in my living room. Thought I'd share some brief thoughts that might be helpful to some, as both of these sets have been found on sale lately (the LG for ~$1K, the Sony for ~$800-900).
PQ: Picture quality was comparable for me on these sets once calibrated. (For calibration I used an old DVE disc for the Sony, and pretty much stuck with the picture wizard for the LG). Satisfying black levels on both (coming from a Panasonic plasma, I was prepared to be disappointed, but they are both good). Sony had better vertical off-axis viewing, whereas the LG had slightly better horizontal off-axis viewing. Some very minor light bleeds on both units--the Sony on the bottom left, the LG on the bottom right. But they are only really noticeable during certain scenes when the room is very dark and you're in 'critical viewing' mode. Neither bothered me enough to go through the hassle of re-packaging and gambling with another unit. I found both sets looked best with pretty much every single picture enhancement turned off, except local dimming for the LG.
Smart TV: As opposed to some, I didn't find the LG's magic remote gimmicky in the least. Instead, I found it very useful and powerful. The Smart functions on the LG just outclass the Sony in every way. Not only is the Sony poorly organized (as mentioned by myself and others previously), but the interface is clunky and old looking. I also found the DLNA performance on the Sony to be slow, especially navigating through folders. It also didn't handle a number of files in my library. The LG had *far* better DLNA performance, including high quality 1080p files and 3D files as well. The LG has been so good that I have disconnected my HTPC (a trusty Revo 3610) for now and will see whether I can go without a separate box for my library. Note: I'm using the built-in media streamer on my Synology NAS, which the LG found and can navigate fast enough as to not frustrate me. It's tough giving up the sleek XBMC interface on the Revo, but simplifying the setup and reducing how many things are plugged in is a good tradeoff IMO.) The LG also has a great voice-input command system and powerful search function, and it interacted well with my TWC set-top box. The LG has better apps (though Amazon VOD is notably missing so far), and faster performance. Web browser was good for quick lookups, and having PIP with web browser and TV or whatever other input is a nice feature.
Streaming: No lag for me (as opposed to some user reports and at least one professional review), but I have good internet speeds and a fast router (ASUS N90 or something like that). Hulu PQ was great, as was Youtube (even though I couldn't figure out what setting the Youtube app was using, it looked at least 720p quality). While clunky, the Sony didn't lag streaming content from Hulu or Amazon either. However, the zippier performance gives the LG the edge.
Looks: Both are great looking sets. I think the LG looks a *little* sleeker with the thinner bezel but both are attractive. I'm using these on stands and like the Sony's lower overall profile compared to the LG.
3D. Nemo looked jaw-droppingly nice on both sets. I think the Sony had a more subtle 3D image overall, and it had more ability to tweak 3D effect levels (though the default ones always looked best to us). I think I may slightly prefer the more subdued 3D on the Sony compared to the LG, as the latter was more pronounced and jumped out of the screen more. The glasses on both were comfortable, but I did like that the Sony's would clip on to my eyeglass frames.
Gaming: This was a concern for both. but on the appropriate 'gaming' modes both sets worked well. I would give the Sony a slight edge here. On gaming mode, the Sony had no noticeable lag and very low jagginess in fast-pased FPS. Coming from a plasma it was a slight step down but I have to say that I like the LED PQ for gaming a lot. The LG had no noticeable lag either, but the jagginess was more pronounced when panning quickly across the screen. I changed the input label to 'Game' and 'PC' for the HDMI input from my PS3, and they are similar, except that PC seems to lock out even more picture options than the Game input label, and requires its own calibration. I tried all the various picture modes on the LG to make the picture improve. Eventually, I found that not all games are equally demanding, and many games look really stunning with all the processing turned off *except* for the motion setting, which I prefer to leave on minimum (called 'smooth', I think). I find this really makes things smooth and gives the image a nice punch on the LG without negatively affecting input lag. On some games, though, turning on the motion processing lead to some image artifacts. In Borderlands, for example, the lowest 'smooth' motion setting makes panning look nice but the crosshairs in the middle of the screen jitter / skip around during fast pans. Nonetheless, on other games (even FPS games), leaving the motion setting on minimum really helps. I'd encourage you to try it. (I never turned on any other picture processing setting to keep the input nice and lag free.)
Overall: Despite being more expensive, and even a little too big for our current setup, it was an easy decision keeping the LG. The Sony was nice and had better / more stable performance for gaming, and we would actually prefer the 50" to a 55", but it just felt like a lot more basic, utilitarian unit when placed next to the LG. If Smart functions / DLNA mean nothing to you, then you might as well go for the Sony because the PQ was just as good to my eyes. But if those are features you value to some extent, then the LG just feels like it's in another category.
Hope some of you find this helpful.