From what you are saying here, it looks like all that they really do is give you a greater viewing angle... If thats true, why would they ever be needed? Its not like more than one person is normally looking at my monitor...
The biggest advantage of IPS is that it gives you 8 bit color. When running in 32 bit mode on a computer, that means there is a 1:1 mapping between what the video card is sending to the screen and what is being displayed. 8 bit means you have 256 variations of a shade. (Grey scale will truely be grey scale). TN panels are 6 bit, or 64 variations of a shade. So that means if a color is 128, 129, 130, 131, being sent from the video card it's going to have the same color on an TN panel, while IPS will display a different shade. TN panels will dither the colors between the shades, and will "swim". If you look very close to your screen with a TN panel and look at the pixels around it, you will see that the pixels are always shifting colors. It also creates banding between the colors.
http://www.lagom.nl/lcd-test/gradient.php#gradient-h.png
See that page for what I'm talking about. On my monitor here at work, if I look at the gradient closely (about 6 inches away from the screen) you will see that is always flickering (swimming) but from farther away it looks normal. The swmming can cause eye strain.
Not only that but the colors on IPS are more vibrant. The reds are redder, the blues are bluer, on TN panels it looks like things are dirty like there is a slight film of grey on the entire screen which reduces the vibrance of the colors.
I bought an IPS TV at home, and it looks so much better than my 27" TN panel, I'm now considering buying a 24" IPS and downgrade on the size to get a better color reproduction.
FYI: As others have said, since IPS is displaying all 8 bits, the monitor takes longer to transition from color 255 to 0, while TN panels go from 63 to 0, which cause them to be a bit more laggy.