Extremely good deduplication means that for the core set of very important data I backup to Tarsnap the costs are negligible. I imagine the math is probably different if your data is changing more frequently. I for instance use other services to manage my video and photo libraries but my accounting databases, critical documents, etc are backed up to Tarsnap.

I have been using Tarsnap for a decade and not only has there been minimal availability issues there have been almost no issues of any kind that I can recall.