I own a Kindle 2 from Amazon. I have HUNDREDS of books on it.
At work, I have evaluated the Sony and Barnes and Noble Nook, as well as some other readers. I have found that it's a 3 horse race, and that only these three are worthy of purchase.
In ranking order it's:
1. Kindle
2. Sony (any of them)
3. Nook
Nook is a distant third at this time.
Here's what I think about them:
1. WiFi is not really needed, but a cellular data connection is very useful.
2. eInk is great - but it's got a long way to go before it's cheap enough for a nice newspaper sized device.
3. Tablets will likely not get anywhere near the battery life or light weight of an eInk based device.
I would buy a Kindle if I was buying a first eBook reader.