Since the Surface Pro is a full fledged computer, it should be able to run Dragon Naturally Speaking so one wouldn't necessarily need to use a keyboard sitting on their lap.
Also, we need to keep in mind when reading all the negative reviews that that fruit company GIVES writers hardware ( and in many cases, cash payments I believe ) to write negative articles about anything that doesn't have an I- in front of it's name.
It's strange (for me, that is), but I don't find myself even attempting to type save for when I am at a desk. In fact, it is even more strange that I don't find myself attempting to use the Surface as a tablet! I seem to using it more like a laptop, that is to say, I almost always use it with the keyboard. I am not sure MS wanted this kind of use thouh.
But to respond to the OP's question, I'd imagine that while it would indeed be possible to type on one's lap, it would be at least a bit uncomfortable like trying to type on a traditional laptop (well, t least I always seem to find it uncomfortable).
I was also inclined to use the Surface as a laptop initially because it works so well that way. After a short time I forced myself to go tablet mode only. After the first couple of days I have yet to switch back to being a laptop mode user (not really doing any serious productivity on it). I have been using it flat on my lap (if using on my lap at all) and using the on screen keyboard for the most part. It definitely works great in both modes and it you don't need to type 40+wpm or use Excel the on screen keyboard is fine for most basic text entry tasks (surfing the net, FB, email, etc.).
I have my netbook or my work laptop, if I want a laptop. I have used my Surface on my lap and it works fine, but I prefer to use it as a tablet most of the time. I tend to use my Surface for general browsing and being a bit more mobile out of the office, so the tablet form factor fits well. I find the split on-screen keyboard to be pretty good for most tasks, but I'm not usually working heavy data entry tasks either. I used Excel on my Surface the other day to record surface finish results while I was out at a vendor reviewing product. Would not bother with the laptop for this situation in the past and would have just written everything down on paper and enter it on the laptop when I got back to the office. Saved me a bit of work, but did cost me sometime explaining the Surface to the owner of the vendor I was visiting. He was intrigued with the Surface RT and what I was doing.