All the above comments seem quite helpful ... I would add my two bits
though.

I am no countertop expert; but, I did a countertop a few weeks ago.
What I did was to go to Home Depot and buy the tops with backsplashes
already cut at 45 degrees. I then took these tops to a professional
countertop shop and had them "tie" them together. They cut an
elaborate series of notches (3) along each edge and tied them together
with mechanical "clamps" after applying appropriate glue. These clamps
were on the underside, and were covered over with a 3/4 inch thick
piece of particleboard. This gave me a very secure joining of the two
pieces at the corner. I paid about $100 for the total work ... joining
of 4 separate pieces of countertop (two L-shapes. They came out just
perfect. Yes, the corners of my walls were pretty close to right
angles before I started. In my case the expense was well worth it.

Bob