my honest opinion is that majestic is the best for backlink search but very expensive. right now seo spyglass seems to be best match when it comes for cost/benefit.
and yes there are monthly search engine maintenance fees with paid seo spyglass version - read up on this more and research other tools with similar features - most come with recurring fees as well but thats a whole other discussion.
whoever said that you get locked out of the free version when you dont pay - I am 99% positive this is not true, the software reverts back to free if you don't pay ONLY if you remove your registration key and name from the program - otherwise you get a message that your live plan expired - so to co back to free remove licence key info
and then you can use the free version for as long as you like since link assistant has no time cap on the free tial. another issue with the free spyglass version is that it only supports yahoo, icerocket and some other search engine and thats it - so you are kind of limited in terms of backlink sources.
still even with recurring fees factored in seo spyglass is cheaper than majestic and backlink analysis part is absolutely phenomenal. that's why guys at backlinksforum and trafficplanet, admins including, use it and talk about it extensively, which is where I last heard that link assistant is working on adding majestic to SEO spyglass to compensate for Yahoo. I am guessing the new feature will come in by february but so far seo spyglass works with Yahoo and I get backlinks as usual.