I own a Merlin Annual Pass, and don't live too far away from Alton Towers, so I am lucky enough to go quite a bit throughout the year. The end of July is always a busy time of year. On a personal level, I do find the Fastrack system an absolute rip-off, but for some reason, a lot of people seem to buy into it. If you want to go on all of the rides in one day, and do not visit the park very often, then it might be worth investing in. Bare in mind that the likes of Submission and Enterprise are less popular, and queues for those rides go down as the day goes on; in effect, it may not be worth using the Fastrack for those attractions.
I have purchased a Fastrack pass during the Scarefest events for some of the most popular rides such as: Nemesis, Air and Oblivion as part of an £8 package. In order to save money, it might be worth doing this rather than blowing £45. Its up to you though how much you are willing to spend to avoid waiting in long queues.