Just started on sitagliptin today (Januvia), and I have seen a massive effect on my BG. My control had been pretty good, but I'd been keen to reduce spikes at dinner time (which would rise to 8-ish, but for all other meals I will peak at 7) - my consultant was loathe to prescribe me fast-acting insulin, and recommended this stuff instead. The price looks fairly similar, so I'm kicking myself for not asking why he was so dead against Humalog or Novorapid instead.
Anyway, just to try it out I ate a ridiculous amount of random carbs this afternoon, including: a pint of regular milk, a punnet each of cherries and strawberries, a protein bar and 4 crackers. I should point out that I rarely stray near any of these things, none of which I'm able to eat in any sort of moderation. But here's the kicker: my BG an hour later reacted as if I'd eaten a light snack. It also fell far faster than it normally would.
It's tempting to stay on this, but I have a bad 'no such thing as a free lunch' feeling, particularly given everything I've read about the pancreatic cancer risk. I realise they've only retrospectively found tumours in corpses, but that does nothing to allay my fears.