Who has said it was as good as Sopranos? I know I don't think it is. Although BB is very good. Sopranos is in a league of its own with a few other great shows. It's not fair to compare other shows to Sopranos, TAGS, MASH, Cheers, Seinfeld, Frazier, etc etc. Because you will consider them a fail if they don't measure up.
All of your questions about Walt's character and his personality switch very quickly? It is summed up in the title of the show. I think that is supposed to be the irony of it all. Feasible and realistic though? Maybe not.
I've heard it described as "better than Sopranos" several times. Perhaps not here.
It's not really that I consider other shows a fail, but there are certain shows that set standards that the rest should aspire to.
I like (and occasionally watch) lesser shows. Dexter, which I like a lot isn't Sopranos-level but I enjoy it. I did like Dexter better when he was just eliminating people that needed it and not having epic showdowns with people like John Lithgow or Tom Hanks' Son/Admiral Adama.
Spartacus, which I didn't like to begin with, grew on me with the complete debauchery and a pretty decent nugget of story and intrigue wrapped around some laughingly bad dialogue. It's not Sopranos level either.
Bad is not quite Dexter but it's better than, say, The Glades which I do unfortunately watch hoping for improvement.