1)They are best at damage over time spells.
2)As party members they are fillers usually, not main tanks, not main healers, and not main dpsers, but very important fillers IMHO. They can choose theyr pet according to what the party needs most, void for more tanking(usually partys with only one tank, but even partys with to tanks that are not ideal tanks like low level one and/or not very skilled ones), succy for more dps(party with good tanks but low on rogues and mages), imp for more buffs(very good in a party with already a warrior, a priest, a rouge/mage and a paladin/shaman/druid that does tanking or dps as needed), since imp job is to buff it is more usueful in a 5 man party, so if you are going in less usually you will not prefer him to the other pets . They will make healthstones for everyone that are similar to potions but with separate cooldown and can place a soolstone on partys preist/shaman/paladin so that if he dies can come back and reiz all the party. The dots wont shine to much on damage meters since a lot of enemyes will die before they are fully used but will be precious for bosses letting you make a good over all damage without risking to take aggro off main tank and without getting your mana low how cuold happen with a mage. Remeber that your crowdcontrol spell fear should never be used in an instance.
3)usually you will be able to aford for a good price a 20slots soulbag and that will be very good to bring full when you begin an instance.Remember to wand, after using your dots most of the times its not worth siking your mana and taking aggro by spamming shadow bolts, so its better to wand off something than just doing nothing.