It would appear that its innovative design gave it a 360 degree traverse and higher hitting power than the comparable Pak36 & Hotchkiss 25mm guns. The fact that it remained in service throughout WW2 is testament to the gun's usefulness. It was displaced by heavier ATGs as heavier German tanks appeared but remained a deadly opponent against the types of tank it was designed to combat. Thus in the Far East it was more than a match for all Japanese tanks.
I am about halfway through the build of Zvezda's 2pdr model gun. I have to say that I have enjoyed the build and been amazed at the sci-fi look of the gun as it's gone together. It must be those tripod legs which give it a very strange look. But then it's only strange because we're so used to the post 1942 ATG orthodoxy.
Here's some pictures of the gun without its shield and crew:
Secret Vickers Plans
So, to answer the title's question. Was the 2pdr ATG as crap as legions of half-assed articles and books have stated? No, it wasn't. It was a gun of its time and in its time was an effective, reliable and useful gun. The fact that it remained in front-line service against tanks it was never designed to fight is no fault of the gun's. Look to the politicians and the planners if you want someone to blame.
PS: In response to Chris Kemp's query I found this on the Lone Sentry site: 2pdr ATG