I've been on Betaseron for a lot of years now...at least 15, maybe more (I'd have to go digging through my medical records to see for sure when I started.)
I have to say I feel better, since starting it. Am I attributing that "feeling better" directly to the Betaseron? No.
I'm feeling better because -- before I started the B, I was having 2-3 exacerbations a year, each of which caused significant impairment, and it took a long while to recover from each one..only to have another. Now, since starting B, I had just one, about 2 years into starting it, and none since.
Are any signs of MS gone? No. In fact, I've had a slow decline -- I was walking (with forearm crutches) 15 years ago; now I use a wheelchair full time. I have bladder issues that worsen sometimes, seem to recede other times (controlled by medication). I have bowel issues. I have weakness in my limb. My vision goes "wonky" from time to time. And so on.
But I'm not having exacerbations. So I'm not going through those periods of intense flare-ups of symptoms, feeling ill, being hospitalized, having to work my way out of them.
I'm "feeling better" because I can plan a bit better for the future. Not having flares means that maybe I can do something next week..or next month..or next Fall...and not have to worry that I'll be hospitalized with a flare. I'm "feeling better" because, while I still have MS and tons of stuff going on, nothing much new has come along, so I don't have to "learn new tricks" to deal with this disease.
I'm "feeling better" because, while it's only a small bit of control, I've still taken control of this disease as much as I can, doing what I can to slow it down (taking the med), and it seems to "be working".
The drugs may ... or may not...stop progression, or slow progression. As the others have noted, MS is a very peculiar disease/disorder -- we each experience it differently. The drugs (Avonex, Betaseron, Copaxone, etc.) don't treat symptoms; there are other meds you can try for some of those.
One last thing...for me, "feeling better" has come about because I've reached a level of acceptance, of understanding. I know (for the most part) how to balance having MS with other stuff I want to do in my life, so I'm not always fighting the MS so that I can get about my day, but instead have learned to live with it. Which I've done for 25 years, now..and counting...
...I am not a doctor, nor health professional, and don't pretend to be one, here.....