Nobody knows for sure!
My belief is that they're probably less valuable than links from on-topic pages.
Even if that's not the case now (and I think it is) - then it's quite likely to be true in the future: If Google haven't implemented it yet, then it's the sort of thing that they will implement as soon as they can get it to work. After all, common-sense dictates that if you weight on-topic links more heavily than off-topic ones, it's likely to improve search results.
Having said all that - I still think off-topic links do have some value (it's too hard for a machine to work out what's on-topic and what's off-topic to disregard "off-topic" links altogether.
So my system is: Get on-topic links wherever possible - but it's ok to get some off-topic links too.