Missing English-Greek exercises along with much of the information you would need to complete such exercises!
I just started browsing through Machen at Markos suggestion, and it definitely goes into much more detail right off the bat on some of the issues, like accenting, that seem very tedious in the beginning but come in very handy down the road. I got all the way through Mounce without having any idea where to place an accent. That might not seem to matter when you are simply reading, but as soon as you try to form a sentence of your own in Greek you'll realize how important it is to know what governs accent placement. That is just one example.
Also, jumping right into some verb forms before even dipping into nouns is a big departure from the Mounce approach, who doesn't get to verbs until chapter 16. I'm not sure which approach I prefer when it comes to verbs.
I've only browsed Machen very briefly so far, but it seems like it might be difficult to do both of these books side by side due to how differently they introduce the various paradigms and other info. Perhaps going through Machen after Mounce would be a good bet? I suppose chronological differences between 1st year grammars will always be an issue though.