I've not experienced skin breakage so no venom and cannot give an opinion on pain. Biting is rare for home keepers but there have been a few animals that have become aggressive, making tank cleaning difficult. I have found the ones here to be generally territorial and will warn you about "messing with their dens" but not attack. Some "play" rougher than others and it is important to keep your hand away from the beak area, regardless of how well you know the animal. If your fingers smell/taste like supper, it may easily be mistaken and get nipped even by a docile animal. Like most, if not all, wild animals, octopuses are not domesticated/domesticatable. USUALLY, if they start to put too many arms around your hand, you can gently touch the back of the arm and be released. Sometimes this takes a bit of manipulation as you only have 5 fingers but TYPICALLY, that is more than the number of arms trying to attach. The first time an octopus reaches up and touches you is usually a shock and a thrill for both parties. In spite of knowing you should not jerk your hand away, instincts on both sides usually end up with this reaction. I still unconsciously jerk a little when Octavia sneaks up on me while I clean her tank.

She has a habit of doing this and it is quite comical. It appears that she is just letting me know that I am invading her space and that I better behave as she is not aggressive but definitely sneaks up on me but keeping well camouflaged until she reaches out to touch. If she was inclined to bit, she could probably do so.
Do suckers feel very different than other touching experiences? I would have to say yes. Most live octopuses are not slimy and are very soft to touch (there are exceptions to the slimy but you are not likely to acquire one. Dead animals are very slimy.). The suckers are not rough but are definitely suckers and a strong animal can leave temporary marks on your forearm and back of your hand (I have never had marks on the palm area).
It is not unusual for an octopus to accept light petting between and in front of the eyes once they are not afraid of your hand. This area is above the arms and not an easy grab to attempt a bite. I don't believe they can actually see you pet them but some will come to their keepers for a small amount of very gentle handling. I see this most in older animals and suspect it has to do with scratching an itch kind of feel on older skin.