I have tried it and it does work well. May take your FBGs down 2 to 3 points eg: 13 to 10. I had no obvious side effects and in fact I liked that it stopped my irritable bowel type symptoms which I get from metformin. Having said that, you need to note there is what they call a black box warning on it from the States linking it to Pancreatic Cancer.
I didn't want to take it but like you, they wouldn't let me take the insulin. I did take it for about a month (Works best for FBG taken at night) but then a friend sent me a link to the UCLA study which for me brought it stunningly into focus. Google Januvia/Onglyza and UCLA, it should come up or go to a website called BloodSugar 101 and there is a link there.
The study autopsied diabetic patients who had died from head injuries or strokes. Every diabetic autopsied that had taken Onglyza for a year or more was found to have Pancreatic Tumours (every single one) so that's a little scary but what makes it even more scary is that every diabetic autopsied who wasn't on Januvia had no tumours.
To put it in perspective, the tumours were I think all benign? but it is a fact that benign tumours can be a precursor to malignant pancreatic tumours. So maybe scary, maybe not. But for me, what is scary is that pancreatic cancer is possibly the one cancer you do not want to get. The survival rate is under 10% (from memory) and you generally get very little time from diagnosis to demise. It is just one of those very nasty cancers as I understand it.
I am no expert at all, this is just the decision I came to. I gave it a month so I could say to the doctor I had tried it and then I demanded the insulin. They try this approach because it does seem to work and it is a simple pill to take. One small pill at night - very few obvious side effects for most people. But one potentially very bad insidious one which may be more common than they currently realise.
When I switched to insulin and it hasn't been long (maybe 6 weeks??) I did see an even better response but my FBG seems to be responding to day time rapid insulin rather than overnight basal (No idea why that way round exactly). I have put on a little weight with the insulin so there are pros and cons there too.
No one can decide for you except you. Nothing is totally safe and if you are gestational only? you wont be taking Januvia long so that's another possible consideration. We could probably scaremonger about most of the medications open to us and there are always risks but for me personally, that was one I couldn't accept. I have school age children.