I've seen alot of people complaining about leveling difficulty in a bunch of different threads, and alot of complaints have been coming from assassin/shadows in particular about how it is "impossible" or "so hard its not enjoyable" to level 30+ since this last build.
I'm making this post to explain what build I'm using and why I think assassin/shadow leveling is actually extremely easy even after this latest build. I'm sure they will lower the difficulty slightly for release at the very least, but if they do not this build will guarantee work for leveling at the current difficulty level.
This is mainly for people who are having alot of trouble leveling, but if you are leveling fine with a different build maybe this one will still be faster/better/easier or maybe you will just like how it plays better. Maybe you won't like this build and think yours is better. Either way this is just my opinion on what is fastest so take it or leave it. =p
I will be using assassin skill names and talent tree names because that is what I played most, but for shadows it easily transferable.
The Build - http://www.torhead.com/calculator/skill#200rs0ckrszZZf0cMb.1
You will be leveling as a "tank" so you will have a dps companion for the majority of your time leveling. Doesn't really matter what companion you use for dps but I like Revel the best once you get him mainly because he is ranged. Don't bother with the droid he is terrible.
You will spend the entire time in your defensive charge/technique. You use full dps gear except for the shield gen which is a must have.
You will put your first 22 points into Darkness and then the rest into Madness. Notice it only goes to level 43. This is because at 43 you have all the talents you really need for leveling, and it doesn't matter what you take. I would get duplicity next so you can do Maul in group situations if you are dpsing them.
What makes this spec work is Shock/Project, the energy regen, tons of Thrash bonus damage, and the fact that you get basically a passive 9% crit increase 100% of the time. Energize in Darkness makes every shock a crit 100% of the time. It does massive damage even in tank stance. Walk up to mobs - Thrash Thrash - Shock - Thrash - Thrash - repeat forever. You will do good damage but also mitigate enough damage with your tank talents that you can afford to use a dps companion. You breeze through mobs with only moderate healing needed in most cases. Golds are easily soloable all the way to 50 with this build, I promise you. =p
You can tank if you are trying to do flashpoints/group heroics, but I normally dps'd. Be careful and make sure you have a good healer if you tank because as you get higher in levels you start wanting to have tank gear. You start getting tank/dps gear options from quests. Dps gear is your first priority but you can pick up tank gear if you want to make it easier to tank. Switch to lightning charge for dps in groups and if you plan on running alot of group play instead of solo then get duplicity for maul spam sooner than level 43.