I have two such rotated displays on my work desk side-by-side to get a 21" x 23.5 combined screen. The problem I get is that viewing angle on average displays is pretty week in the vertical angle (great, so nudge it up or down an angle) - but if you put it in portrait mode, this acute viewing angle really hurts - just moving your chair from one part of the desk to another can expose it.
Unless you're going with IPS displays, the portrait mode of a monitor is somewhat compromised.
I used a TN display in vertical position for a while and it sucked. If you moved the head just a little sideways the colors completely changed, and after a couple of hours I started to feel nausea. My doctor told me it was because the eyes are seeing different images (each eye saw the display at different angles, and because of the poor vertical view angles the brain received different colors and light and had to compensate - like using polarized 3D glasses). Now I use a combination of vertical + horizontal 16:10 IPS displays and it's way better. I mostly use the vertical one for web browsing and doc reading.
Unless you're going with IPS displays, the portrait mode of a monitor is somewhat compromised.
I still hate 16x9. Please give me back my 4:3.