This topic is going to attract a few opinions (what in yachting doesn''t) so I will throw in my 2c worth (adjusted for currency conversion and inflation + GST).
Cement boats have exactly ONE virtue to offer. They are cheap. Cheaper to build then either GRP or cold molded timber or alloy.
Of course int he modern world, the hull is only one part of the overall investment that equals "yacht"...and that means that quite frequently, cement boats represent a dollar value that can never be recouped. Because the spars, blocks, sails, electrics, electronics, winches, engine, etc etc etc are just like those found on a "real" boat...only bolted to a hull that a majority of sailing folks will never trust beyond staying afloat at the dock.
Cement boats suffer from a couple fo issues that give them a bad rep (some of which may not be justified). Many of these boats were "home built" as there was a huge fad for them soem years ago. Quality control is thus a bit...errr...patchy, at beast. They have lumps and bumps and irregular thickness in the hull. The mix is variable, so some areas are more brittle then others (and cement boats are ALL more brittle then GRP or wood).
Some mixes of cement are not as water proof as they ought to be and suffer capillery leeching. A lovely state in which you are living on board a seive nd trying to throw the water back overboard faster then it comes in. your major ally in this battle is PAINT. If this thought comforts you, then go for it.
Also linked to the above point is the reinforcment of the cement. It is chickenwire.
This means that in a grounding, and inpacts or whatever, lots of little concrete hexagons fly from your hull, and leave behind chickenwire to hold back the sea.
My favourite worst case scenario though, comes form the possibility of lightning strike! Imagine all of the chicken wire reinforcing suddenly going incandescently hot and blowing off the cement that surrounds it. Up the creek without a paddle does not begin to describe the situation!
So what are cement boats good for...well, you can buy a 40 foot cement boat for the same price as a 26foot GRP boat an use it as a live-aboard in oone of the nicest marinas you want to live in and that is it. Your "floating" concrete apartment by the water. Just don''t try sailing it too much or in too hard a weather.
Oh yeah, cement is kind of heavy too...so you will have a boat that weighs about the same as a steel hull...but with absolutely none of the advantages. (even your compass will have devation issues like a steel boat, because of the mesh in the hull).
Anyway...that is my opinion on the matter. I am more then sure that there will be cement boat officianados along any moment to defend their point of view.