I have every perk in Conjuration and can safely say, Necromancy isn't that great at all really, it starts off fun but by the end it's just pretty damn pointless, you can conjure up a Storm Thrall incredibly easily wherever you like, yet with Dead Thrall, you rarely ever find anything decent that it works on, and when you do, it usually disappears immediately upon fast travel.
Actually it's not that bad I guess, it just doesn't seem anywhere near as powerful or as convenient. Just wish Dead Thrall worked more things, something like Frost Trolls would be great, and that they didn't disappear upon fast travel.
Necromancy with dead thralls easily blows away storm thralls, but you have to put effort into it. Dead thralls will use whatever weapons you put on their corpse before you raise them, some will even use summoning staves. They will also use armor that was on their corpse when they first died, but you can take that stuff and enchant it and upgrade how you want, then stick it back on the corpse. Another benefit of the dead thrall spell is that you get another storage mule, two if you have the twin souls perk.
The fun part about necromancy is the experimenting and collecting. The way the dead thrall mechanic works really enforces roleplaying a necromancer. Dead thrall corpses don't turn to ash when they die, you can walk a thrall to a place that doesn't reset, like a player home. The result is being a creepy wizard who collects dead bodies in the basemant of his cottage.
I have an idea that would make necromancy summoning tree perfect imho: make raise zombie, reanimate dead, revenant and similiar spells actually level capped thrall spells that worked only on humanoids, the master spell would work on level 50+ mobs. Then make the skill level 70 perk one that allows you to raise non-humanoids and push the hit point bonus down to 30.
Those who are really interested in the power of the dead thrall spell
HAVE to look at this huge topic on gamefaqs: http://www.gamefaqs.com/boards/615803-the-elder-scrolls-v-skyrim/61115903
Losing thralls on fast travel is a bug and a workaround is discussed in that topic.