Now I've tried both, the only buy 10 with 'buy 10' and only buy 100 with 'buy 100' are fab - what the game needs is this and a 'buy max' button. Best of both worlds :)
As many before me have said... Why have the buttons been broken? Buy 100 should act as buy max. One simple change has sucked most of the enjoyment out of the game....
A little too easy, but very enjoyable (and when these kinds of games get too hard it sucks all the fun out of them so much better to err on the easy side!)
If you don't mind playing level 1 over and over a few times, plug all the money into health and extra men and you can be fully upgraded in about 10 minutes....