I used the 100$ free credit they once gave away (not sure if they still do that) and was rather underwhelmed by most instance types, the big ones were nice but very expensive (unless you expect the price of the coin to rise extremely). Most of my experience were with XPM and QRK, mined lots XPM with a dirt cheap 20€/month VPS, until the hosting company found out and cut the performance (host-unlimited, pathetic company and I don't say that because of the performance cut, which was expected and is a major reason I don't like VPS [variable performance unless you go KVM but then you can get a root for the same price]; getting the server took weeks).

TL;DR: I think one root mining 24/7 makes more profit for the same money in the long run. IMO of course.

Those spot instances seem like a nice thing where you decide what to pay, are they new?