I have the S&S AHR Meridian retractor, which is in our Astra. It's easy to use (once you read the instructions!), but with the Astra, the car seat angle is too deep so you have to put a rolled up pillow case (or something similar, hand towel perhaps) under the base so it levels out more (which is recommended in the booklet, nothing unsafe). Also, you have to cut a small hole/space in the boot-cover piece (that thing that goes from the back of the seat to the back windscreen - what's it called?!) and have to buy an extension for the strap ($20 from RedCross).
Hmm, sounds like a lot of work, but all of the above only costs $20 and some time.

I noticed you can order them (brand new ones) online for quite a bit cheaper than the shops....so shop around.