Does it matter about the sites niche you get a link back from? I thought link building was about getting just links, and don't worry about the site.
You should read this great article, it'll tell you all you need to know about backlinks: http://wiep.net/link-value-factors/
When I buy a pair of shoes, I expect them to fit me perfectly. If your link is on a unrelated website, then the traffic will be mostly untargeted. Everything link-wise must fit with what you want to deliver. Eg: Shopping link - sends shopping visitors Gift site links - sends gift type visitors bear in mind that just because you send/receive targeted visitors, they still need to convert. The wrong visitors will never convert, so why invite them to your site. Yet I see this every time eg: "Get links - ANY links as they are great". then the same people wonder why they don't make sales Ofcourse gaining any links en-mass is easy, you just link to anything and everything, heck I was doing that 2 years ago and got 50k worth, then I realised it wasn't working well enough and changed linking strategy. Sure I get less links, but they are on-target and better converting.
I realize this has been some what covered, but to add my 2 cents to the question.... Google (from what I have read on blogs, news releases and other forums) is a big, big fan of relevant unique content. Thus, if your backlinks are there for the purpose of improving search engine rankings so as to try and get natural traffic from the search engines then it is very much in your interest to make the website linking to your relevant. There maybe nothing you can do if an un-related site has linked to you, but if its in your control, then the link is worthless or possibly harmful to your pr. There are some websites which are considered "bad neighborhoods" for example. Sites full of pretty meaningless links, or particularly spammy sites. Link to, or from one of these, then you and a whole lot of other sites could find yourself losing page rank very fast. I know this is not the case in this scenario, but I use it as evidence to substantiate my claim that the content on the page linking to you does seem to be of relevance to the search engines. When ever I go into a niche, or build a website, I look at the top ten competitiors, and check where there backlinks are coming from, what sites are linking to them, and what anchor text is being used to link to them. I then count how many backlinks the site has in total and I attempt to reciprocate the process without directly duplicating the process or content. I hope this makes some sense, and is of some use. I'm only a newbie here, so I could be epically wrong. Anyway, rave safe, its a jungle out there.
I'd also like to apologize for some horrific spelling grammar and spelling in my last post. It was speed-written. My humble apologies.
Demonic! linkbuilding is a long time consuming process and involves lot more than science and technique than just have a simple backlink to your website. Successful backlinking campaign can get you higher position in search engines results and than traffic, which results into sales in the end.