The best example that comes to mind is Dell Computer, who rose to prominence on the back of a multi-year ad campaign that compared their products to Compaq's products. These ads were not only straight product feature and price comparisons, but were even fairly sarcastic and unflattering to Compaq. As long as they didn't actually provide false info, though, and provide proper acknowledgement of any trademarks, the ads were legal. I don't see any difference with web advertising that is up front and avoids falsehoods or trademark violations. Having said that, of course, that doesn't mean that a deep-pockets competitor couldn't sue anyway with the intention of causing financial distress and winning the battle before it even reaches a judge.
Meta tags are somewhat less clear.
I don't recall who prevailed, but my guess is that Terminix lost. Metas that relate directly to the page content combined with appropriate trademark acknowledgements should be OK, in my opinion. But free advice from a non-lawyer is worth what you pay for it, so consult legal counsel for more definitive info. I just don't see the point of mentioning your competition in the meta tags. I had one of our competitors do this, once contacted they had to remove it.