I like it and I've done it a lot in my reviews.
Sometimes, when I don't have the compared products, I have no frame of reference. "It's softer than a bloorgenvirter, but firmer than a glurch". Huh?
But when I DO have the toys being compared, it's instantly informative. I totally agree with bratcat about visual comparisons (photos).
I also like when they make a visual comparison with a common, everyday object. And I also find it helpful to verbally compare with everyday things when describing texture, firmness, etc. For example, I think it's generally more useful to compare a dildo's firmness to an erection than to another dildo. But it's easy enough to do both types of comparisons, so why not do both?