Pros
I worked as a software engineer. There are of areas to work on from the website to the warehousing. They have an attitude of doing virtually everything in house. I got very good experience there. Teams are small, work isn't bureaucratic and there are opportunities to be very innovative. Typical 40hr working week and flexibility about how you do your hours are good.
Cons
If you're very driven, you might find it a bit relaxed / casual / not strongly motivated. The office is in Hatfield. If you live in London, it's fine for a while, but most people get bored of the commute out. Your cost of commute is high, yet Ocado probably pays less than you could get in London. Being in Hatfield people don't stay around for drinks after work, but race back to London. This means there is less office social life.