I think these brush heads are great, they're more comfortable than the heads that came with my current toothbrush and are cheaper to buy too. The bristles are quite stiff, the heads are marked as being Philips standard texture and I would say that is a little bit harder than a medium manual toothbrush. They never hurt my teeth or gums but I remember when I first started using them they felt much rougher than I thought they would, this is good because it gives a good clean and when fitted to my toothbrush (SoniCare FlexCare) the motion created by the motor stops the bristles from feeling so rough against my teeth somehow.
After brushing my teeth I rinse the brush head under hot water then remove it from the handle and store it on the small pillar built into the charging station. I use my toothbrush twice a day so the head isn't in the open air for long enough to get dusty or anything like that so I have never thought to buy a cover, my mum doesn't use her electric toothbrush every day because she prefers manual brushing and she has had to buy a case for her brush heads because they sit still on the side of her sink for too long between uses.
A section of the bristles on the brush head are coloured blue and the blue fades while you're using the brush head, when it has faded down to the plastic of the brush head then it is time to replace it. This might be before or after 3 months, I don't replace mine on 3 months to the day but use these blue bristles as a guide and think they are quite accurate because by the time the blue has reached almost to the bottom the bristles are starting to look a little bit worn anyway.
These replacement brush heads come highly recommended by me, they only fit certain models of Philips SoniCare toothbrushes but they will fit most of them and anywhere that sells the heads will have a list of the models they can be used with. I would not recommend you buying eight brush heads without trying them so your first pack should be the smallest one available even though that will work out to be more expensive, if you like the heads you can buy a bigger pack next time and if you don't you won't have wasted too much money.
Diamond-shaped bristles in a high-density bristle pattern / Suitable for Philips SoniCare FlexCare+ / FlexCare / HealthyWhite / HydroClean / EasyClean and DiamondClean handles / Push-on brush head easily slips on to the handle / Reminder bristles fade indicating when to replace / 4 pack.