I used to get IPL about 8 years ago when I was still in high school and was dealing with acne scars. What I noticed it would do for me is dry out any existing pimples, and with the correct amount of downtime it would help to lighten the scarring.
IPL has improved since then, now when you go in the procedure is much shorter but they also use an applicating gel that helps to pick up the frequencies I believe. When I go into them now they take about 15 minutes but if you see the right person they will give you teh full treatment for the hour ;) .. meaning some type of mask prior to it, a massage, etc, and then the short treatment.
The results are pretty immediate for me now, but the great thing about IPL is even though you leave with your skin looking dramatically brighter, softer, more even... you also continue to see improvement over the next few weeks. This treatment is great for people who are just dealing with mild discoloration, very fine lines, and maybe an occasional breakout. It will really add a brightening / lightening affect. But remember to use sunblock regularly!
I don't suggest this to people looking to treat deep scarring, or deep lines. However it may be the way to go if you suffer from breakouts! Along with a great regimen at home. Just remember not to expect anything to happen right that second, be patient with your derm or your esthetician and it will pay off :)