We have enjoyed our Vizio M420SL for almost 1 month. The picture is fantastic, sound is good, and the features are fantastic.
I read some complaints about the performance of the Smart apps, but we have had a great experience with YouTube, Amazon Instant Video and Hulu Plus.
There aren't nearly as many apps as you'll find on a Roku or any of the Sony products (BluRay Player or PS3), but there are a good core group (Netflix, Vudu, Hulu Plus, Amazon, YouTube).
We have connected the TV to our WiFi (802.11N) and it works beautifully.
The only complaint I have is that the buttons on the remote are very stiff. This is probably good for the QUERTY keys since they're on the underside of the remote and the stiffness prevents accidental pressing while you're using the standard controls on the front. Otherwise, it's pretty easy to navigate.
We're also very pleased with the size and weight. It's right around 2" thick, and only weighs 30 lbs., making wall mounting very comfortable.
Overall, we're very pleased and sure hope it works this well for years to come!