What should I do when I see bad web design?

I quite often come across badly designed websites. But I always wonder, should I do something about it? Should I send an email to the company and politely tell them about the usability issues I encountered?
How would someone react getting an email out of the blue telling them their site sucks? Even if I could phrase it nicely, would people take this as an insult? Or as constructive criticism?

I have enough trouble giving constructive criticim to Cecil. They are relatively well equipped for dealing with bug reports and helpdesk enquiries, but usability problems don't seem to be handled so well. A typical interaction goes like this (note, to protect the innocent, this interaction didn't happen exactly as described here, but this is a fairly typical type of response):

Me: "I notice that your display is truncating the names of files to 20 characters, even though there is enough room on the screen to show 100 characters or more." (Note: I didn't think it was necessary to spell out in detail exactly why I would rather see the full file names, I assumed that would be obvious.)

Them: "Thank you for reporting this issue. This behaviour is by design in order to conserve screen space. We suggest that if you want to avoid your file names being truncated, then you should keep your filenames to less than 20 characters long."

Then, I'm pretty sure that if the problem is recorded it all, it is entered into their bug tracking system as a priority 20 (lowest priority), where it languishes at the bottom of the list of 5,000 or so bugs they have to fix.