Vantaggi
ESL has many different clients using Erlang to solve interesting problems. If you're lucky, you can end up working on something you enjoy. It's a good place to meet many like-minded people, especially at conferences that ESL organises. Moreover, there are many opportunities to travel as a lot of clients are not from UK. It's a good starting point for a junior Erlang developer to get some Erlang experience.
Svantaggi
The management tries to get as much money out of each contract as possible regardless of the quality of the delivered service. Majority of the projects is not successful - the projects are not managed properly, there is no long term planning, even the short term planning is amended on a daily basis. Most of the work is done chaotically, and in many cases, close or after the deadline. Some decisions of the management truly ruin the projects. A developer who started and been working on a particular project for a couple of months is often sent away to work on something else and is replaced by someone else who is not familiar with that project at all. Developers are often disturbed by the sales team, that lacks technical knowledge, asking technical questions. The sales guys try to get as many contracts as they can because of the commission. It results in not telling the whole truth or deceiving clients and even developers in order to make them start working on the given project. The employees, especially developers, don't stay at the company for a long time, sometimes just for a couple of months. The salaries for such city as London are small. There are not many senior developers to learn from anymore. The management doesn't care about what the employees think at all. It's quite sad as this company could have been doing much better. Erlang is a wonderful technology, but in the wrong hands. ESL hasn't been able to create a real product which would show off the full potential of Erlang, instead they must rely on consulting.