I explicitly try not comment about competitors or even perceived competitors of the companies I work for. For instance, I have some very specific feelings about the 6a LJ merger and what it means and how I think it will turn out. But I don't write about them publicly. There are a lot of reasons for this, but I honestly believe that when people write about their competitors, credibility suffers. There are exceptions, of course, sometimes you gotta defend the mothership from the slanderous crap out there. But defending what you do is different than taking potshots at other companies.
The funny thing is, whenever someone takes a shot at google, justified or no, I desire, I want, I need, to respond, but those are often the times I try my hardest to not do so. To spend time on my work which, in the end, will discredit those who hate us faster than any transitory post on my blog, or a comment on theirs, might.
When I was at VA I saw this all the time. We had competitors who spent some time and money on trying to discredit our work there, it was all very evil. I fought the good battle/etc for VA, and I had some reputation riding on said defense. If I had wasted it saying that competitor x or y were worthless, then it would have been just that, a waste.
On the other side, not commenting (Snap! I got him!) , or judging my own company too harshly (Is he trying to appear trustworthy?), can be dishonest in their own way too. In the end all the blogger can do is use their best judgment. Judgment seems to be in short supply in the blogosphere though. As does perspective. This won't change in 2005.
Speaking of which, I have to point this out: Blog spam and abusive behaviors in comment streams are not new in any way, shape or form, we had comment spam in Slashdot in 1998. Go read the code and learn something. (This paragraph from the "Damn kids, get off my lawn." department.)