Hi Flan - I have always used 100mg vaginally (sequentially, 12 days a month) and never had any issues, though everyone's response is different. The only annoyance for me with it (sorry to be graphic) is that as it dissolves I do "feel" it during the night - feels a bit like having a lot of discharge, but it's the Utro. No matter how high I whack it! But it's a minor annoyance. I feel no side effects at all in terms of tiredness etc.
Another option is to use it rectally like a pessary if you can't get on with it vaginally. Not a lot of people do this but it is another option I haven't yet tried.
There's no way I could take it orally - even 100mg orally leaves me like a zombie car crash the next day!