I played with this a couple of years ago when I did my first wedding (that sounds like I've done loads lol, I should clarify it was the first of two!), my findings were as follows:
I have a Pantone Huey, so YMMV with anything else calibration do-hicky wise.
1. Calibrating cheap screens is worse than useless. This is because their blackpoint has a colour cast which can't be calibrated out. Calibrating the screen will therefore make bright colours look good, but the darker the colour gets the more it becomes inaccurate. Photos with any sort of dynamic range go very screwy indeed. After a great deal of fiddling I eventually concluded that the laptop screens were less bad uncalibrated, since at least that way the error is more or less constant with brightness level and therefore you can get used to it. You just have to get into the habit of religiously checking your monitor alignment if photo editing on the lappy.
2. Calibrating expensive screens is MUCH better, but for most practical purposes largely unnecessary. I bought myself a 19" HP with a PVA panel. There are better screens, but I was poor and this was the cheapest I could find which didn't run a TN panel. The difference between the HP and my old TN equipped flat panel is night and day, and the TN desktop screen was already better than the laptop one. I ran the calibrator on the PVA panel, and yes it made a difference but the HP is so close to right out of the box that I don't find it worth the time and effort.
In short my advice is don't bother calibrating unless you've got a good monitor to start with (something with a PVA or IPS based panel), and then only if you're doing something REALLY colour critical or you believe your monitor is displaying significantly untrue colours out of the box (I'm given to understand that I'm lucky with my HP and even good monitors are often not that great OOTB).
The tricky bit is finding a screen to buy which is definitely PVA or IPS, the vertical viewing angles are a good indicator, with PVA and IPS panels generally showing almost no brightness variation with viewing angle (which in itself is a good enough reason to buy one for photographic work!).
Annoyingly I don't know of a laptop with a non-TN panel at any price, they just don't seem to be made
