3) Long duration always on items are generally underpriced in 3e. In particular, you are much more likely in my games to find an item that lets you cast a spell on yourself X times per day, than you are to find an item that grants you unlimited access to that spell. A 'ring of invisibility' that lets you be invisible all the time and has no drawbacks is IMO, something that shouldn't be showing up at all before very high level and should be priced under the item creation guidelines accordingly. In particular, the 3e item creation rules don't take into account balancing factors of the spell like the normal length that the spell lasts when determining what the spell should cost when made effectively 'permanent'. There is relatively greater advantage in making a spell that lasts rounds last 24 hours compared to making one that lasts hours last 24 hours. At lower levels, a 'ring of invisibility' or 'ring of flight' that let the wielder use a spell once or thrice per day is much less abusive and more likely to be balanced with the rest of the campaign.