I am on about week three with my shield tab and it has been up and down. The up is that gaming is AMAZING. I don't even have a controller yet, on order from Newegg where they are in stock.
Now for the bad. I am on my second tablet. The first one I got from Newegg and had problems almost out of the gate. The best way to explain it is problems going to sleep and waking. I would push the power button and nothing would happen, push it again and it would freeze. When trying to wake up it just wouldn't do anything at all. I would reboot and it would work fine for a while. Talked to Nvidia customer service and after uploading videos to youtube and such they filed an RMA and sent me a brand new tablet in about two days!!
Now having the same problem to a MUCH less degree. After gaming, doing searches, browsing it has trouble going to sleep and waking. Once an app starts, no problem at all. No lag at all with gaming. JUST going to sleep and waking from sleep. Whenever there is a problem I restart the tablet and it moves smooth as silk for several hours until it does it again. I've tried closing apps all the time, doing RAM optimization with third party apps, nada. Seems to happen when I quickly go from running an app to sleep then awake again. I would blame in myself but I did the same exact actions with my Nexus 7 with no issues at all.