Is it comfortable to hold for long periods? Depending on your hand size, it's a perfect fit, but it is heavy, so weight might be an issue for extended periods, just depends on the user, but comfort is top shelf.
How nice is the screen? Battery life? Buttons and triggers? The screen is excellent, but the bezel has wasted space, so that can bother some. Battery is the BEST. Depending on use, it can last 10+ hours. Buttons and triggers are console quality, but the d-pad could be better(but it's not bad).
Besides retro games, are the Shield-specific games nice? They have some Tegra 4 specific Shield games, and they are mostly very good, but there just Android games with spruced up graphics and physical controls. Which can be a world of difference depending on the game.
Finally, can you use your Google Play account with it? YES
When the Shield was first released, Nvidia had a demo finder on their website, as GS's around the country had demo kiosk set up. I'm not sure if demo units are still out there, or if the demo finder is still active, but give a look and see. "N" says that the WiiU needs to be experienced to be understood, but I've played it at stores, and it didn't move me to rush out and buy one. There is nothing there to understand, except for the HUGE tablet controller, that doesn't really add anything to existing games(just another gimmick).
I think if you hold the Shield in your hands and use it, you'll love it. Anyone can see how this was designed to offer the best experience possible in a handheld form and is serious about doing so. At it's current price, it's still the best android gaming option available.
BTW, the speakers are mind blowing, no joke.