Pros
They may not remember me since I wasn't there long (hint, I shared a first and last name with one of the owners), but Apax gave me my first full time job (1099 hours) when I graduated college with a CS degree. I do feel I was able to learn a lot. The company culture is great. The owners/devs as well as senior level devs were all a pleasure to work with. The variety of projects was nice as well, I particularly enjoyed the Django/Angular stuff when I was there. The experience gained at Apax has played a part in advancing my career substantially in the last 10 years.
Cons
The only con (if you can call it a con) was the pay at the time, but keep in mind I was fresh out of school technically. I was hoping to eventually get brought on full time but I guess at that point in my career I wasn't at the level they needed, and while I think I would've been eventually, I came across an opportunity elsewhere and since I had loans and was recently married I had no choice financially but to accept the other position. I personally held no ill will towards Apax and I don't think they did towards me either. I've grown quite a bit since then as a Software Developer, and do think if the opportunity to ever return arises I would be happy to work at Apax Software again.