I have two - the iGo keyboard (but it's no longer available new, though there are some used ones on Amazon) and the Freedom iConnex BT keyboard....
Amazon.com: Freedom i-Connex Bluetooth Keyboard (iPhone 3GS/4 - iPad - HID Devices): Electronics
It's specially for the iPhone and iPad and has dedicated buttons for things like 'Home', 'Volume' etc.
It comes in a faux-leather pouch and is powered by 2 AAA batteries/cells, which last about a month of heavy use. There's no setting up....just go to 'Settings', 'General', 'Bluetooth', put the keyboard into discoverable mode (recessed button that needs a paper clip to press), the iPad will 'discover' the keyboard and suggests a pairing code that you then need to type into the keyboard. Then you're good to go.
The keyboard goes to sleep automatically after a period of non-use (though there's a switch too, to switch it completely off). When it's in Sleep mode it disconnects from the iPad. Pressing a key (sometimes several keys), wakes it up and the iPad 'recognises' the keyboard and you're off again - i.e. no need to type in the pairing code again. If the on-screen iPad keyboard had popped up, as soon as the BT keyboard is recognised that keyboard recesses itself automatically.
The keyboard is plastic and feels flimsy. It's not completely full-size either but it's not noticeably smaller - i.e. you soon get used to typing on it. The keys are certainly not as nice as the Apple keyboard but that's the compromise for making it very light indeed and very small when folded.
I've had mine several months now - typing on it at this moment - and had no problems as far as reliability is concerned. However, I am careful with it. The faux case is quite substantial and can certainly protect it from an accidental drop.
Bottom line - it's a trade-off between incredible convenience (it's so small and light that you're never tempted not to take it) and 'full size'. I learnt to touch type as a kid (misspent youth - I should have been out on the streets!) so I find the on-screen keyboard OK for very limited use.
Tim