I thought it was well-put-together. I didn't love the viruses -- instead of feeling like an enjoyable challenge, it just sort of felt like a chore, eventually a "click on another tab and hit a snooze button" chore. I don't think I needed more than 1 or 2 levels of the virus thing, I don't think I ever used more than two charges before one came back.
I like the idea of a market and timing it, but in this case I'm not sure it added much -- given how slowly the market moved, and how often upgrades would give you big increases, it seemed like the best move was just "sell regardless".