I went to a 19" from 17" about a month ago. Both monitors are high quality and have excellent pictures. I did not really "need" a new monitor, but I got a good deal on the 19"er so I bought it. Following is my experience:
* The 19" being larger required me to raise my chair or lower the desktop for comfortable viewing.
* The 19"er made me feel like I had the screen right in my face.
* The 19"er requires more eye movement to see everything in full screen mode.
* Some applications are more comfortable viewing in a smaller 17" size window instead of full screen.
* The 19" burns more electricity (more heat) and is heavier.
* The 19"er allows for better use when running several open windows at one time, however if you run this way, you might be better off with a 21"er.
* Games are nice on the 19"er, but some games only go up to 800x600 or 1024x768 so they will be more grainy when open to full screen.
I'am still using the 19"er and I like it, but it took about 3 weeks to get used to it. However, I could have lived with the 17"er and still be happy. If your monitor needs replacement, consider a 19"er, but also look st how it will be used. If you do buy one, get a good quality one. Buying a cheap one with a poor picture will end up being a regret.