Most recently they have introduced the option of a subscription for viewing better quality images - personally I am comfortable with that as I recognise that research is not really what they are there for. But I use the site almost daily for reference and I can understand the cost can be difficult to justify if use is not so intense. Being realistic, it is an incredibly heavy site and must be very expensive to run in server terms. I doubt they sell enough maps to finance it from that.
The bad news is that in the last few days I have noticed some recent server failures as of old.
There are several other sites offering similar information - they do not tend to be as easy to navigate but the output quality is usually better.
PS - I think this disussion would be best in the "Historical" section so am moving it across.