Monrovia's fine, so long as you have people on the ground to point you in the right direction. The airport is very small and hence it can take a while to get processed. Try not to arrive immediately after a scheduled flight.
The drive into Monrovia can take a while - the traffic can get very bad. There's a few decent hotels and restaurants.
Security in general is fine - the UN are there in force. But of course there's the usual petty stuff that you will find anywhere in that region.
In my view, Liberia is vastly better than the truly awful DRC.