I have a large window seat in a bay window and I love it. It's extremely practical as I have cupboards underneath and they fit a huge amount in - ideal for when people pop round and I have to quickly shove my mess away before I go to the door! I have proper upholstery cushions on the seat and this makes a huge difference as they are extremely comfy - the cheap ones you get from places like Dunelm are fine, but just not as comfortable (they always give me a sore bum despite my significant padding ). However, they were expensive and I bought the fabric on EBay to try and keep the cost down. What are you thinking of doing/having?
Yes, we have an L shaped one. Table is 120 x 70 (small kitchen table) and we've had 9 people eat round it, albeit some of them were very small. Love it because without it, our kitchen would be too small for a 4 seater table.
Definitely have storage underneath. It is worth fitting flooring all the way underneath. The thing I hate about it is how difficult it is to get in and out of the seats round the table legs. Get a pedestal table, and if the seating is L shaped keep the table lightweight so it can be easily adjusted. Remember to allow space for people to get in and out - for us this means the bench being longer than the table.
I created an upholstered bench for a client, using ikea metod drawers and and a fully upholstered top and back. They us this with a metal legged Formica topped table and additional chairs as their main dining space.