I've had a few done. The ones where you just walk up to them only tend to be brief such as trying some eye colours out.
It also tends to very much depend on how busy they are. I remember having to do a mad dash to Selfridges to get some false lashes put on as they were so busy in HOF when I went in, only just got them done in Selfridges too as they were so busy in there as well even though they had so many staff on.
Trouble is with places like MAC, they provide a lot of advice as well as the demonstrations and such they do.
If you want to make sure you can get it done properly I'd strongly advise booking in and maybe having a wander round (or look on the website) to have a look at colours and looks you like.
The makeup artists are great, as I said before, but they do tend to favour the strong makeup look which is not for everyone :)
I have to say, MAC have some fantastic makeup artists working on their counters.