It's an obscenely high risk game (I wouldn't even call it an investment). There's possibility for a complete loss , mediocre return, or a low possibility of high gains. Everybody selling the hardware is against their own interests. If you had a machine that made money, you wouldn't want to sell it for less than your potential profit.
Manufacturer wise they come in all shades of dodgy. BFL have such a long order book (year and a half at this point) that the devices they are delivering cost hundreds of thousands in BTC initially, and will never make even their own cost back. Avalon was initially making very profitable products, but suddenly stopped because well, their competition supposedly threatened them. ASICminer sells products that will never earn their cost back. KNCminer is unproven outside of a single FPGA prototype. Bitfury makes nice products and ships them on time, but they're hard to come by outside of Europe. All the others are either blatant scams or appear to be.
It's an extremely touchy game to get involved with. I've read enough that I would never take the risk with any of these companies; people get screwed over time and time again. There's people who decided to take out college loans and buy miners with, which just sort of demonstrates the mindset you need if you want to "invest" here.
Disclosure: I own one ASICminer device that I kept around to keep my Raspberry Pi busy.