As an investment, I'd suggest that Wii U systems/games probably aren't a great buy.
But I agree with what others are saying: if you see a game you want to play at a good price, don't hold back. There will probably be a period where pricing really bottoms out in a year or so, but by then many of the less common/popular games will already be hard to find and rising in price. If you want a collection, start shopping sales now and keep an eye on stock levels.
Personally, I'd really like to pick up an extra console. I have concerns about how long my Wii U will last, and fixing/replacing the gamepad will probably be a pain if it ever conks out.