I lived there for 15 years and raised my now 19 and 6 year old there until we had enough and moved to Buckingham. Much better!
My eldest was robbed of his phone in the park on the housing estate where we lived, we came across needles in the nature reserve near my house and on the path next to the school my son attended and this, along with the volume of housing being built but few extra facilities all added to us wanting somewhere different.
Buckingham is lovely! Train from MK to Euston (35 mins), train station opening in Winslow (5 or so miles from Buckingham) in a couple of years, great walks, outstanding schools, a nice town centre and good choice of supermarkets (Tesco, Sainsbury's, Aldi, Waitrose) or pop to Bicester or MK for further shops. Friendly community, surrounded by beautiful countryside, always something on.
We've now been here 6 years and have never looked back.
If Bicester is a definite for you, we had a good experience with Bure Park Primary School and The Cooper School for secondary. Great transport links from both stations. Sports centre with pool. Range of churches. The town centre has a good range of shops (a few empty) and a large Sainsbury's or the new Tesco Superstore. Bicester Avenue has Wyevale, Laura Ashley, Hobbycraft etc. Some great restaurants too.
A friend moved into, and then quickly out of, the new Kingsmere estate due to lack of community feel and a few petty crime issues but I guess you might find that anywhere and the estate is still being built. Every estate in Bicester has its good and bad points. I guess it comes down to price, gut feel and school catchments.
We enjoyed 13 out of our 15 years there. It just wasn't right for us personally in the last 2. Had family who lived there 30 years and love it and will never leave.