As other reviewers have said, yes, insurance is 100% paid, but coverage is not the best (a few buy-up options are available, but they're quite expensive). If you rarely/never visit the doctor, and don't have any prescriptions on a regular basis, you might be okay with it. This is the only reason I'm hesitant to give a full five-star overall rating. But, as noted above, the fact that there even IS a fully-paid option is pretty unique.
Because Liaison is similar to a staffing agency, your options for advancement, promotion or raises are not always clear. Some of their people do get hired on by their clients, though there's no defined timeline or guarantee that will necessarily happen (I guess some clients do not hire outside the contract with Liaison, so just make sure you're clear about your assignment, and know what your long and short-term goals are before accepting the job).
And I don't know if it's quite a con, but once you start your assignment with Liaison, your day-to-day contact will be exclusively with the team at whatever company you end up working with. Liaison is not going to be checking-in/following up with you very often (and they are clear about this fact when you're brought on board). Which, if you want to be left alone and do the job you were hired to do without a regular "So, how's it going?" conversation, this is a perfect arrangement. But if you crave regular feedback or ongoing coaching of some kind, just know that it won't come from Liaison.