A lot of freedom and respect given to developers to work the way that is best for them, to use the technologies they think are best for the job. A strong culture of well-planned projects means you are rarely rushing for deadlines. Opportunities to learn from really good developers. Great Christmas parties!
Cons
It's possible to get stuck working the same project with the same technologies for a while if you don't fight your corner.
This is the rare software consultancy that values customer satisfaction, rather than customer cashflow diverted towards consultants.
Huge latitude in choice of technology. Get to work on some interesting projects with interesting people.
They look for people with an interest in theory. If you do, then you'll find the office chat highly stimulating.
You can make a difference.
Cons
The usual risk of being stuck in a project you don't believe in, I suppose.