Hi,
I'm French and I did my undergrad in France. However I am still applying for PhD and I am definitely not a marketing specialist, so take this with a grain of salt.
ESSEC is definitely a notch below the other schools you applied to, which are quite comparable. They all are part of the 10 or so semi-elite European schools that that only clearly lag behind INSEAD & LBS.
About ESSEC: it is still predominantly a teaching school. Its reputation comes from their masters, which are considered (in France) on par with HEC. However their emphasis on research is very recent and their PhD has only been accredited in France a few years ago (they previously had agreements with public universities in order to issue doctoral degrees). Tbh I am not sure they have a strong record (or any record) of placement out of France until the last few years. The research groups are very small, since at least half of the faculty only publishes in French journals, doesn't publish anything, or does not even hold a PhD...
On the flip side, the quality of the doctoral program is apparently OK. The best placement you can hope is probably HEC, which is great. A growing number of PhDs are placed abroad in respectable schools. For those who stay in France (probably the majority), they mostly find tenure-tracked positions in decent French business schools or in public universities.
To sum up: if you absolutely want a position in a research university or if you want to go back to Northern America (assuming from your previous posts you're from the US), decline this offer (the only student they placed in the US may be an INSEAD PhD dropout: http://appsrv.pace.edu/lubin/faculty/departments/showFacultyDetail.cfm?Name=Imran%20Chowdhury).
ESSEC will most likely place you in teaching-oriented schools in Europe, where your teaching load could be 2 or 3x what you can expect in a competitive US b-school, even if you will be expected to publish somewhat.
On a more practical note, your salary will probably be in the €40-50K range (unless you end up at HEC or a comparable school), which is enough to enjoy a comfortable, upper-middle class living in a medium-sized European city.
It's up to you to decide - that's probably a life choice down the road.