I've only flown once with the tablet plugged in. No smoke came out, which is very important-- these devices all run on smoke, once you let any of it out they never work right again.
After flying, I used the tablet to watch some videos posted here on BCP via the airport wifi. Checked the charge later and it was at 97%, it was at 75% or so when I plugged it into the airplane earlier so apparently the 12V-5V charger worked OK.

My tablet is 16GB, works fine with the Avare app. It also has an internal GPS which is a must, otherwise you need to hook up a separate GPS like a Bad Elf or whatever. I see some of the 7" tablets are only 8GB, I think they would also work with the Avare app except some of them do not have the internal GPS.

Here's a link to the Avare webpage where they talk about tablet requirements.
http://apps4av.com/avare-overview/
It says any device running Android 2.2 or newer.
I just checked my tablet, it says "firmware" (??) takes up 4.77GB, apps take up 2.17GB, and downloads take up 2.79GB, plus some other small stuff. It also says 8.01GB available, which doesn't jibe with the other figures. But I'm no techno-dude, so who knows? FWIW I also have the MyRadar app on it. Plus maybe some other apps that came already loaded on it?

Thanks hotrod! I'll be searching for at least a 16gb tablet. Also appreciate the reply about the charger.

After I got looking, I discovered there's actually lots of 7 & 8 inch tablets under $100.
I got this one because I figured 16GB was better than 8GB, and it had the internal GPS. Some don't.
The specs for the one I bought say it has the "Android 5.0 Lollipop" operating system. I've seen other cutesie names for Android system editions- seems like 5.0 or 2.2 or whatever would serve just as well, but that's just me.

Amazon offers used tablets also. Just make sure it's got the internal GPS or you'll need an external GPS gizmo. And if you want to run the Avare app, it needs to be an Android device. There's other free apps, FlightPlanGo for one. It's available in either Android or ipad versions, as are some pay apps like Garmin Pilot.

Update: it's been about 6 months since I bought my Insignia tablet. Been using it for navigating via the free Avare app, no problem for the first few months. Then a couple months ago it started having trouble locking onto the GPS satellites, and occasionally losing signal lock in flight. Just recently I borrowed a G-Sat Bluetooth Bluetooth GPS receiver from a buddy, surprisingly that didn't help. Also recently, I started having trouble connecting with wifi's. The last straw came when I decided to uninstall the Avare app and reinstall it-- the uninstall went fine but I couldn't log onto the wifi to download the newer version. Back to Best Buy to return the tablet. Day or two later, I ordered up a Samsung 8" tablet to replace it. Twice the price at $200 but still a pretty good deal. It was delivered yesterday so I've been messing with it today. Downloaded the Avare & MyRadar apps and got it all set up. It boots up way quicker, locks onto the GPS satellites way quicker, stays locked onto them even inside my hangar...just performs better all around than the other tablet did (even when it was new). Plus the Samsung just seems like it's better quality. So my suggestion is to not buy an Insignia tablet. The bottom line is that the least expensive option is not always a better deal.