I've always had one - on third now in nearly 40 years. I never intended using the dryer much - dry outside on clothes line, or on airer inside, but wanted one for the times things come in a bit damp, or need something dried in a hurry. Agree that they don't suit a full load of washing, but from what you say MissA, it sounds like you wouldn't be doing that.
As far as I know, all the ones now are condenser type. As the machine is already plumbed in, they use a flow of cold water to condense the moisture from clothes, so no extra plumbing or pipes are needed. However, there is a newer type of condenser type, which works like a dehumidifier and has a refrigeration unit to condense the water. Cheaper to run apparently, but more expensive to buy, and another bit to go wrong.
As you're in a flat, the advantage of combined washer/dryer is mainly space saving. If you decide to go for one, look at reviews first. The one I have now is a Miele, but it's the most expensive brand. Cheaper ones may be just as good.