Yes, dirty glass or glass that's not easily cleaned will make a your pix very bad.
As to whether B+W glass is better than Hoya or Tiffen glass, it depends on your whole setup. If you shoot a good lens at low ISO, you might be able to tell the difference. Otherwise, what's the point?
Look at it this way: is glass that costs $10 going to be as good as glass that costs $100? Sure, the difference may be 5-10%, but like in sports (eg 100m athletics, where the winner beats the runner up by 0.1 s or less), like in all other fields, getting the last few % of performance takes a lot of $$$.
i'm not talking about its build or how easy it is to clean the glass...
but when compared to brands like hoya or tokina will it really make a difference in picture quality?
Last time there is someone who do some kind of test. IIRC, Nikon filter give slightly better picture quality than B+W. Hoya/Tokina is slightly below B+W. Don't ask me (before someone pretend as a newbie ask for pictures), I am not the one who doing the test.
BTW, it is true psychological may bias you. I am bias to German for optic.
One thing about B+W filters is that the ring is made of brass instead of aluminium like Hoya. I had a few problems before when I tried to remove the Hoya UV filter during a shoot to replace with a polariser. But the aluminium ring will bite into the lens thread and make it more difficult to remove with the fingers. Even more problem to remove polariser because the outside ring rotates!
With brass ring filters like B+W removing filters are a lot easier as the brass do not bite into the lens thread. Just my two cents.
The trouble starts when everyone is able to view their pix at 100%, which is like printing a pix 1m wide. So, you start to see all the flaws in the picture. When you view or print at realistic size, you cannot see those noise, CA or other minute flaws.
When you compare among those multi-coated ones, hardly any difference. Color cast? Well, it is only when you need such color accuracy that it makes a difference. If not, under normal circumstances, you can hardly see it at all.
We all know that those Hoya HMCs are hard to clean. But then, how often do you need to clean them?