I have been on Mex for about ten years now and I too am completely reliant on it. I am 25 now and have been on the same dose since I was 15. However, my brothers both have Beckers too and there doses have increased over the years. If I go a day without it I am completely stiff the next day but while on it I would say I am 95% 'normal' functioning. I notice too if I am active the day I miss a dose ( I do a lot of warming up first) that the next day my muscles are very sore.
I too have been worried over the years that this drug is too strong to be taking daily. I've read negative comments about heart damage from various places, however you cannot believe everything you read on the Internet. My brothers and I go once a year to see our specialist and get our prescription renewed and I asked a few years ago to have our hearts checked out because I was worried. We had heart monitors on for 24hours and the results came back completely normal. I check my blood pressure often and it too is normal.