This small restaurant, tucked away behind Pentonville Prison, is definitely worth seeking out. For me it was a first (but I hope not last) taste of Ethiopian cuisine, and I found it extremely enjoyable - for £16 pounds ahead we had enough food to keep us very nicely full, with an Ethiopian beer to drink.
Served communally, and eaten by tearing the unleavened bred into little 'parcels' one fill with meat or vegetables (these were particularly nice), eating is an enjoyable and eating experience.
Kokeb is a small restaurant (6 tables, I think), and clearly the lovely and welcoming owner has a lot to do to serve and (I think) cook as well - so this isn't the place to go for fast food. But if you want to try something different in north London, I can't recommend it highly enough.