Kepler are a good small boutique. They are very niche in what they offer and tend to hire ex-oxbridge/red brick and very high calibre people (the founders are ex-Mckinsey if I remember rightly). Their focus on benefits and remuneration strategy will mean you'll need to think about where you would like to go as a second step in your career and weigh up the pros and cons.
Within their niche they have a pretty good reputation as far as I understand and work with a lot of big FTSE players advising on what the board should pay their CEOs.
Hope that helps and good luck!