You do have to keep an eye on them though as I did find they had laid the CAT cable next to the mains wire in one place and had to get them to pull it and move it so doing it yourself is probably a better way to go.
Before moving in, I had the electricans install additional power sockets and switches and lay down the cat5 cabling in most rooms of the house, terminating it in backboxes which I finished off myself and at a central point in the garage (part of the house) which I also finished off after they neatly trunked the cabling and mounted a 4U hinged rack on the wall.
They hadn't worked with data cabling before so I was very clear on exactly what they had to do and avoid doing, like numbering the cables at each end, not running parallel to mains cables, and keeping the back boxes at least 100mm from power points. It was mostly fine except for one pair of cat5 cables that hadn't been properly capped and ended up being stuck in concrete/plaster, which'll no doubt cause problems in future - shame it's the network point beside my ADSL socket.
If you do get someone in to do the job, you'll need to keep an eye on them. Also, allow for plenty of slack in the cabling at each end, it'll make life much easier for you.