I have the quinny Buzz 3. I was going to get the 4 originally, but when i test drove it the front wheels were all over the place! It made it really difficult when trying to get up a kerb, because as soon as i tilted the chair the wheels would spin! Really frustrating. However i have really enjoyed having the 3.
The back wheels are really wide but after a few outings you do get use to it. I used mine with the maxi-cosi car seat which was great. I also had the dreami carrycot for when i went walking out and not in the car. This was lovely as my daughter was born in December so it was really cosy for her.
We have just moved onto the seat and i love that as well. She seems really comfy in it and falls asleep quite quickly so it must be good!
They are great if you have a small boot (i have a sport ka) and it fits in great, the back wheels come of really easy, making it alot smaller!
The only disadvantages i can think of are the storage basket you buy separately is quite small, but hasn't caused me to much of a problem and you do see them everywhere! It's because they are so popular and versatile, but i am constantly bumping into other quinnys!
I am a great fan though as you can tell!