I have most of the Oblivion plug-ins, so I will give an insight on what I think about them. Some of the house plug-ins have sort of a small quest before you have access to them.
Dunbarrow Cove (Thieves Den) 150 MS points
Summary: This plug-in is desired for the character who is a thief/pirate. It is an underground cave complete with good features. Some items you can buy in there you can't get in the game. There is no guards in sight around here, so you don't have worry about taking off the Gray Fox mask (if you beat the thieves guild). You can members for your pirate crew with certain jobs (fletcher, fence, supplier, sneak). The best is the fence (a person who buys your stolen goods). Not only it is easy access, you don't need to be in the thieves guild to have him. You will also get your own captain's quarters.
Overall: Excellent. I recommend this plug-in.
Frostcraig Spire (Wizard's Tower) 150 MS points
Summary: This plug-in is desired for the character who is a mage/spellcaster. It is a unique looking tower up in the snowy mountains. After you purchased all the upgrades for the tower, you have an underground vault, an altar that summons atronachs (that you can bring along as a tag partner), a spellmaking and enchanting altar (useful if you are not in the arcane university), easy-access portals to all the mages guilds and the arcane university, an alchemy table (temporary boost of alchemy skill if you stand by it), a garden for gathering ingredients for making potions (useful if you don't want to go hunting for ingredients), a library which might have a few skill books, and a bedroom. A special ring will appear in the jewelry box if you bought everything.
Overall: Good. I don't use it as much as Dunbarrow Cove, but it is a great plug-in to have.
Deepscorn Hollow (Vile Lair) 150 MS points
Summary: This plug-in is desired for the character who is a killer/vampire. It is an underground lair nowhere near a town so you can safely live there. After you purchased all the upgrades, you have a dark minion (who will go and kill people for you and take their belongings and stash them away for your taking), a storage area (including a poison supply), a shrine which gives you blessings based on the bad/evil things you do (something that the town chapels will not do for you), a cattle cell which has a victim (you can use him for blood if you are a vampire), an altar which will get rid of your vampirism (can be used over an over again), a study room (has a few skill books), a dining room (does not really have a use but you need it if you want the special item), a easy access trap door, and a bedroom (a coffin, and a desk that has a Nirnroot which will come back after a few days which is useful for the "Seeking Your Roots" quest). The special raiment is waiting for you in the dresser after you bought everything. (Let me remind you to wait till you are a high level, over level 25, so you can have the maximum effects of the raiment).
Overall: Excellent. Very useful and fun.
Battlethorn Castle (Fighters Stronghold) It was free when it was released, but I think it is 150 MS points now.
Summary: This plug-in is desired for the character who is a hero/guard/fighter. After you purchased all the upgrades, you have a bedroom, a taxidermist who will make you trophys, a vintner who will make you special wine (if you find the ingredients), men-at-arms who will follow you as a tag partner (they will stay at the castle if you don't want them to come), a forge which will give you a temporary boost of you armorer skill, a servant who will give you food and drink if you want it (unlimited supply), guards who watch your castle (although absolutely nothing attacks your castle), a combat trainer who you can spar with, a secret treasure vault, some display cases, and a bunch of other rooms with no purpose. The special shield is in the chest in your bedroom and the sword is in the grotto (a few undead creatures down there) The sword gives you a spell if you equip it called dragons breath. (you have to have a high skill of destruction before you can use it).
Overall: Average. Nothing special about this plug-in. I wish you can start siege wars, but you can't. I got this for free, but I think it has a price tag on it now.
Horse Armor Pack - 200 MS points
Summary: Horses are expensive and if it gets killed by angry wild beasts or by falling dangerous heights, it can be a pain in the ass that you have buy another one. The horse armor will protect your horses from danger. (just an upgrade of health, you horse will not be invincible) The first set of armor is free, after that it is 500 gold for another set of armor. If you don't have a horse, the owner will give you her horse but it is a very slow horse. You have a choice between elven and steel armor with no difference but appearance.
Overall: Limited. Only for the hardcore Oblivion players, or if you can't stand your black horse dying after a few attacks.
Shivering Isles - 2400 MS points or available on disc
Summary: Do I need to say more? This expansion gives you so much gameplay that if i explain at all it will take a lot of webpages. More creatures, more weapons, more spells.... endless. Check this site for all the details on Shivering Isles. http://www.uesp.net/wiki/Shivering:Shivering_Isles
The following plug-ins I don't have, so do more research on them.
Orrery - 150 MS points
Summary: I am glad I never bought this. All it gives you is greater powers that you can only have one at a time, and they are not that great. There is a quest involved, but that still does not satisfy.
Overall: Poor. Please avoid this. Buy rock band songs instead.
Mehrunes' Razor - 250 MS points
Summary: I don't know much about this plug-in, but I heard the reward is pretty cool. http://obliviondownloads.com/StoreCa...px?ProductId=6
Spell Tomes - 80 MS points
Summary: It gives you books in random treasure loots that teach you spells when you read them. Does not sound like much, but for 80 points, that is not bad.
Overall: Limited, but not a bad price.
I hope these reviews help you. I will put more info on Shivering Isles and the plugins I don't have. Just please, don't get the Orrery.