I can only comment on IPL machines that I have personally used. I think that the Quantum and Harmony are very good IPL machines and give good results. The Quadra Q4 I would not recommend. You are unable to apply enough settings; this machine only gives two settings which are not nearly enough. The Harmony and Quantum allows numerous adjustments depending on skin type.
I am sure there are many other fine IPL machines out there. These are the only three with which I have experience.