TL;DR: Charlie neeps about typing on an iPad. If you do not own/are not interested in iPads, another entry will be along shortly.
Okay, suppose you have an iPad and want to shovel text into it. What are your options?
Well, the iOS input methods appear to be somewhat locked down. While there are alternative keyboard maps for some purposes, I have yet to see any alternate keyboards such as Dvorak or Fitaly, which I think is a shame. There's some reasonable first-stab speech recognition from Nuance in the shape of Dragon Dictate for iOS, and there is rumbling about iOS 5 containing a high performance speech recognition engine, but if you want to enter lots of text basically you're stuck using a QWERTY keyboard of some sort.
Here's my first cut at describing the keyboards I've tried.
* The on-screen keyboard comes first. It's surprisingly good on the iPad 2 — I think they improved the digitizer's accuracy for picking up finger positions in landscape orientation — but there's no tactile feedback, and some keys that come up frequently in fiction are missing from the default QWERTY grid. Most irritatingly, the apostrophe isn't visible. (You can bring it up by holding down the comma for a second, or by hitting the comma and swiping upwards.) I can do maybe 20-30wpm on-screen, which is okay but not anything like my actual touch-typing keyboard speed on hardware.
* The Apple Bluetooth Keyboard. Pros: it's an Apple keyboard. Cons: if you hate chiclet keys you'll hate this one too. It's very flat, but the back is raised by a tubular rear support that contains AA cells and doesn't fold. The function key row doesn't support most iPad keyboard shortcuts (it's designed for a Mac). Power button at one end of battery compartment gets pressed easily and can get switched on in a travel bag. It's wider (by about 5cm) than an iPad is tall, so doesn't fit in an iPad-height bag. Weighs 350 grams.
* Think Outside/Stowaway folding bluetooth keyboards. These were marketed during the 00s, but are still available on eBay, often in new sealed packaging. Pros: They weigh 350 and 260 grams respectively, and fold, so they're amazingly compact. Good quality scissor-mechanism keys. Cons: function keys don't support iPad, no longer made (so scarce), fragile. Useful if your keyboard's got to fit in a pocket, but that's all.
* ZaggMate/Logitech iPad 2 keyboard case. Pros: it's an aluminium screen cover/stand for the ipad and protects the glass face of the ipad. You can leave the iPad cover behind if you use this one. The keyboard uses bluetooth to connect and is built inside the aluminium "tray". Supports all iPad function keys. Charges over micro-USB. Cons: the keys are about 92% of full size — it's like typing on a 9" netbook, and may be too cramped for some. The arrow key grouping is wrong (the up arrow is offset so it's above the right arrow, rather than conforming to the usual inverted-T cluster: expect lots of typos at first). Weighs about 350 grams. The iPad does not clip into the case upside down (i.e. with the back of the iPad facing the keyboard), so it can only be used as an angled stand. Verdict: Good if you've got small hands and can get used to the arrow key offset. Very good if you need screen protection as well.
* Assorted iPad folio/keyboard cases: AVOID LIKE THE PLAGUE. These most usually use the same ghastly far-eastern rubber bluetooth keyboard unit; it's the worst piece of dead rubber I've touched since the Sinclair Spectrum, and it's impossible to touch-type accurately on one.
* The Targus bluetooth keyboard for iPad. Pros: It's the same size and layout as the Apple wireless keyboard, but it has function keys designed for the iPad (e.g. springboard and power keys that work), weighs a bit less (it's made of plastic rather than aluminium) and it's slightly more compact (the battery step at the back is sized for AAA cells rather than AA). Wider (by about 5cm) than the iPad is tall. The power switch is a recessed slider so it's much harder to turn on or off by accident. Cons: less rigid than the Apple wireless keyboard. Weighs 260 grams, making it a tie with the Stowaway for lightest keyboard.
No external keyboard of the same dimensions as the iPad screen is actually as big as a proper keyboard. The iPad on-screen keyboard gets around this by displaying full-sized keys, but missing some out, which are then accessed by additional stickyshift states which display alternate layouts.
If you want a full sized Mac-like keyboard for the iPad, with the same feel and key positioning, the Targus keyboard beats the Apple Bt keyboard — unless you hammer your keyboards hard, in which case the metal frame of the Apple keyboard will feel less insecure. Both these keyboards are a bit bulkier than the iPad; this will affect your choice of travel bag when transporting them.
If you can cope with a small keyboard with non-standard cursor keys and want a keyboard case, the Logitech/ZaggMate case wins hands-down. With the iPad in the case, its dimensions are very similar to the iPad with a standard cover. Probably not as good for sustained typing as the Apple and Targus keyboards.
If you require something compact at all costs, the Stowaway and Think Outside keyboards will work, but won't give you all the function key access you might want. They're also a bit fragile and no longer made, hence in short supply.
I'd avoid any of the combined folio/keyboard cases except the Zagg one unless you've had a chance to try one out in person and think you can cope with them. The standard rubberized key mat many of the vendors use is absolutely terrible: missing right shift key, offset/mis-positioned arrow keys, no tactile feedback, sluggish bluetooth response, misregistered keys (hitting one key results in a neighbouring character being delivered) and so on.
You might well think that by the second decade of the 21st century we'd have gotten usable portable keyboards. This is, alas, not obviously the case ...