There is bound to be a lot of discussion about gear and clothes, where it makes sense to link actual products to point out our good experience or some design features. Are such links welcome? Sometimes they are much better than a thousand words, but they tend to rot quite quickly and above all I'm a bit concerned about hidden advertising.
Generally if someone's advertising rather than offering helpful ideas it's pretty easy to spot - I'd welcome such links as long as they don't fall into that category.
Perhaps as a rule of thumb though they should be links to the manufacturers product page and not to any specific site selling them?
I agree, I welcome and often use such links myself to illustrate a point or show exactly what kind of product I'm talking about.
As berry120 said, users spamming links is pretty obvious, so as long as it is low key and not posting multiple/many product links, I think it's fine, and beneficial.