I recently stumbled upon a PR7 site that is NOT in my niche. It posts many news articles daily and anchor text is allowed in commenting. There are so many comments that it doesn't seem to be moderated at all. I've heard a few times that links from sites not in your niche are nowhere near as powerful as ones that are. Is it worth my time to keep doing this or should I focus on blogs/forums that are in my own niche? Even if they're only PR1-3? Also, I'm not "spamming" like you see on so many blogs. IE: "Thank you! This information was useful! <a href insert backlink here. blah blah>" I found the website browsing for my own personal interests so my comments are actually intelligent and I know what I'm talking about. I'll just randomly make a word in my 2-3 sentence comment a backlink.
You should also consider outgoing links, if the page is PR7 and too many outgoing links, it doesn't help you. If the site is PR2 and not too many outgoing links, check how many backlinks that site have.
Links in blog comments are not so powerful today as they were before. Most of them are used for spam, that's why Google nearly ignore them. If a blog's author post your link in content it will be much more powerful than comment link on any PR 2 or PR 7 website.
Yes you should consider relevancy, it is very powerful in terms of building quality links, if you are not able to find high pr sites in your niche then leave it just concentrate on those which are relevant to your site whether it is zero or 1 PR would give you lots of link juice. One thing more, if you build links through relevancy then lots of directors visitors are coming to your site......... and while posting comment on your niche related site you may have much ideas about that subject as you are master in that niche because you have a website in the same niche........ it means very quality stuff..... What I tried to say is that quality is much more important than quantity!