I never liked them with DS1. I found it impossible to get anything out so was not sure how a 6 month old would be able to. You have to bite down to get anything out IIRC. You can remove the valve bit so water is free flowing. Means it will spill if upside down but also easier for them to drink. DS1 couldn't drink out of a cup until he was a year old as I kept trying non spill ones. My DTs have been sipping from a cup since 4 months as we don't use non spill (only put water in there so not too bothered about it going everywhere!).
These aren't the ones where you have to bite the valves - I've had those and you're right, they are useless!! It's very easy to suck from them and water does come out, it just comes with a load of air at the same time. They're not non-spill either - they're not free-flowing, but if you hold them upside down then water does drip out.