I've been using SEO Content Machine for years. The other day I looked at my Paypal to see how much I've paid to them over the years, it was roughly $840 over the last 4 years (that's with a lower, old subscription fee). Crazy. So I finally got my hands on GSA's content generator and so far I'm very happy with the purchase. 1-time fee and free updates for life. You can't go wrong with GSA products, I own several and they're all great and very stable.
GSA Content generator is slower than Content Machine, but it works much better, giving you more control over the results you get. I think most people will use this with GSA SER, so let me share what difference I see. Content Machine doesn't fill all fields when you import data fields (or does so rather poorly), whereas GSA's content generator fills all fields. Setting up new projects have never been easier, love it.
So in the long run, it's a great investment, knowing the GSA developers, it's only gonna get better over time.
Yay no more subscription fees!
(sorry for mentioning a competitor, just wanted to give my honest opinion/comparison).