We used these cups from about 6 months old and still do at 22 months, but now looking for an alternative. Generally speaking they are great, we wouldn't have used them for so long otherwise, however a few downsides:
whilst they can be sterilised, daily microwave sterilisation cause plastic valve to deteriorate quickly (manufacturer did replace free of charge). Repeated cold water sterilisation seems to cause the straw to shrink a tiny bit meaning a poor fit that makes it is very difficult to suck up fluid. Not a problem once we stopped sterilising.
They can sometimes be a little temperamental if the straw is not fitting very tightly, they are difficult to use, probably fine for a toddler who can tell there is a problem, not so great for a 6 month old.
Compared to other straw cups the flow is quite slow, this may be a good or bad thing?
The plastic strap on the value is fragile, although you can buy replacement valves it is an additional cost if you are using regularly.
Issue now is our toddler biting through the soft straw part (nice texture to teeth on!) and we are unable to buy new lids. So we are buying new cups each time :-(.
I would highly recommend a straw cup (of any type) for a baby with GERD. Upright drinking helped considerably, and this cup was the only one of many bottles, sippy and straw cups we tried that worked for us.