Not used the D20 but have its older brother the D10. Does exactly what it says on the tin. Great snaps both under and avove water (snorkelling), easy to use (buttons can be pressed wearing thin neoprene cloves). Not looked at the D20 but I would guess that the above holds true for that too.
Used to have an Olympus Mju770 (a predecessor to the Tough range). Took OK snaps but no better than just OK. Problems started just (weeks) after the warraty expired - its waterproofing ceased to be! The customer service from Olympus was terrible - "It's out of warranty." I could have gone the SoG route but TBH, I wasn't too sad to see the back of it and couldn't be bothered (possibly part of the tumour's effects?). I could also have tried getting help from the retailer but since they had lied to me repeatedly about an order I was waiting on just before the drowning trip, f***'em! Personally, I would never buy or recommend an Olympus to anyone - YMMV though!