As far as I know, there isn't a huge problem of abuse in places like Switzerland and Germany (who have recently introduced it).
I think it should be available to those who have a diagnosed terminal illness and are mentally stable. The only problem is, once you allow a certain group access to it, other groups (for example, manic depressives or those with alzheimer's) will start demanding it too. Not all of them obviously, but there's always going to be a severe or bizarre case which challenges any existing laws, then it's back to square one.
I've always been scared at the thought of getting some sort of terminal illness like cancer, and in the back of my mind I feel as though it's going to happen to me at some point. If I knew I could chose to end my life if I needed to, it would be very reassuring. I can't believe that in this day and age, with all the rights we're forced to have, we're deprived of one of the most important ones. It's ridiculous.
The religious argument is the worst though. They never actually talk about the real benefits or problems of human euthanasia, but spout all this BS about how it's against gods will (the same fabulous 'god' who gave you the brain tumour). If you don't like it, simple, don't use the service. Die slowly and painfully with no dignity if you really want to, but don't you dare try to force other people to be denied the service if they wish to use it.
So yeah, I'm totally pro euthanasia, but of course, it needs a lot of discussion before anything happens.
You might want to check out this organisation:
http://www.dignityindying.org.uk/
You might not have heard of them, but they're the ones who do all the lobbying and stuff. Without them, the government would be turning a blind eye completely. I've donated before <3.
Also, this group have appeared quite a lot over the years when cases make the news. Check out their other policies too, they love fags and everything <3.
http://www.humanism.org.uk/home