Having had the same problem with many of my own apartments let me say this:
If the plaster is flaky and soft in more than one spot it's better to take all the plaster off first, then dry wall on top of the lath (lath is the 1/4 strips of wood laid horizonaly accross the 2x4's.
I have pounded plaster off entire appartments and it's just not that bad.
The biggist problme is getting rid of the plaster when your done.
Th Best thing is to rent a bin (tell them your putting plaster in it).
Then with a hammer and a pry bar just lightly bang on the walls and pry. 90% of the plaster will just fall right off with in a few a hours. A 650 sqft APT takes a day or two to completely remove all the platser (the little clumps at corners and windows takes time to clear)and prep for dray walling
Dry walling after this is easlyer then on top of old plaster and will look better.
The extra work is well worth it and your cost is realy the price of the bin to though plaster out. if you have a pick up truck and time to go to the dump, it's mostly your work.