This is the upgrade which provides all UK users with 8 meg download speeds ha ha. You will tend to find that once your exchange is enabled for MAX, it's all you will get. Most ISPs are reluctant to provide "fixed" speeds once the exchanges are maxed.
In practice it works well if you have good line quality, not so well if you don't - it can tend to connect too fast, resulting in lost packets and less actual bandwidth, sometimes a lot less. You can ask them to cap the speed if you have problems.
To find out if your exchange has been upgraded, visit Samknows and put in your details click here