I had my rear windscreen and tiny side windows done with the second lightest tint (I forget what the name of the tint was now, 'light smoke' I think it was). Didn't touch the front windscreen or front windows, apparently there's already an 18% tint on the whole car as it came so adding even the lightest tint to the front / windscreen risks going over the legal limit.
It's good to remember that any windows immediately BEHIND the driver's head level can be tinted any degree (even limo black if you've got parking sensors and night vision ha ha) but the drivers, passengers and front windscreen need to be within that legal limit. Depending on where you live you'll find different tolerance to tints, when I was in Cobham yesterday for BMWOC meet the police were strict and were using light probes and asking people with illegal tints to strip them off on the spot (and promptly return home) - tis to be expected at a car meet though.
My advice, stay legal (get an autocompany / fitters to probe your windows as standard, so you know how much room you have to play with), go for the same degree of tinting on the rear windows and rear windscreen to match them to each other, and don't go too dark compared to the drivers / passenger / front windscreen so it's not too much of a colour contrast.
Here are some pics of mine. A guy based in West London (Shepherds Bush I think) did mine, he's mobile and we used a garage my Dad knows for the location to tint in Windsor (who levied a small fee for use of their space). The guy I used is highly recommended, did a good job and it was £120 for the two small windows and rear windscreen plus another £20 to the garage for the use of their space. It took a couple of hours but more windows = more time.
Rear windscreen BEFORE:
Rear windscreen AFTER:
Sides and rear AFTER (note the difference between mine and Ruby's standard in the middle, and Rohan's on the right (not sure what tint he's got):
Sides AFTER and standard front passenger:
I can let you know his details if you're interested. Just let me know.