Best "real" rating I've seen and picked up was a 4.98. (By real, I mean not a 5.0, which usually means it's a new pax or one with few rides where (s)he's been 5'd.
He tipped $4.
Worst? I've picked up a 4.4 by accident (accepted it thinking it said 4.9) but the girl was okay, no issues. I 5'd her because she greeted me back and didn't slam my doors. Figured maybe she has had a few rough rides before, so I'd help her get up to 4.5.
Honestly think the ratings are a meaningless for riders as they are for drivers. You're trusting a total stranger to judge someome and relay to you whether they are worth picking up. I have a regular customer (exchanged emails) who goes to and from the airport two or three times a month. She always tips me and we chat. She was a 4.2 before I came along, been throwing 5 stars at her every trip. I have also picked up a 3 star (morbid curiosity) and subtly tried to uncover her Uber history. She was very pleasant, said she rides often, I couldn't figure out why she was rated low. Didn't tip, so maybe it was just that old driver "never give 5 stars to someone that doesn't tip!" bullshit. Her fare was over $20 for a 12 minute ride. If I passed her by, I might have sat empty or got a non-surge request. At this point I accept every request and use the rating to judge whether I should kick them out if they behave inappropriately, figuring it is a pattern.