I've been living in San Juan Del Sur for the past 4 months and I have mixed feelings: on one side, you have amazing beaches and great surf spots, some nice cafés, a large expat community (for the better or the worse), a yoga studio, + many services and products that you may not find in the rest of Nicaragua... On the other side, it has unfortunately become party / drug town and I think there are too many loud people who aren't very respectful. Also many foreigners passing through and living here have almost completely obliterated the local and cultural touch of the place. San Juan Del Sur is generally safe and Internet is average (I have a 2 MB connection which is probably the fastest you can have in the whole country).
Since 2013, I've spent about 8 months in Nicaragua. I've worked from many places in the country (Bluefields, San Carlos, Esteli, Matagalpa...) and really liked Jinotepe in the North, but you may feel lonely there if you don't speak Spanish: I did not meet any foreigners there.
If you have more questions about Nicaragua, feel free to ask.