Otterbox Strada for S9 here, and I am very pleased with it. Decent protection and not over bulky. It's a wallet case but the front flap sits within and not on top of the body shell. I use folio style cases and don't bother with screen protectors.
As for the phone, the S9 is great, much zippier than my old S7 Edge and with the case on, it's only a tad bigger and hardly noticeable size difference. The extra screen real estate is a boon. I was tempted with the S9 but wanted the two cameras.
I acquired my S9 on pre-order. Used the upgrade program and part ex'd an old Galaxy S3. Envirofone offered £10 (!) but part ex allowance was £96 - well pleased with that!