I'm sure someone made one of these posts already, but having slowly mastered the skill, I figured I'd share a few thoughts.
Overall, the skill line is fun. Being a pickpocket can find some interesting items. Looting safeboxes can really take casing out an area, which was probably the most thrilling portion of the skill line. And most of all, you can make a lot of gold. Some things feel off though.
Pickpocketing - The risk vs. reward isn't there. I've heard some people say they've found purple motifs, but I've not seen it in any easy or medium mark, which are the only two you have a guarantee of looting after mastering the skill line. That means taking the chance on hard marks. So using a sample size of 100 hard marks with a 90% chance of being successful (always waiting from the 70% chance), I was caught 47 times, and never found anything over blue quality. Marks are unlimited with travel, but it's very slow.
Armor/Weapon Stealing - The risk is extremely low with much higher rewards than pickpocketing. Finding gear worth 40 to 60g each is very easy, and you can easily make 5000g a day doing this. You can also do this within maybe 10 to 15 minutes.
Looting Containers - Another very low risk with much higher rewards. Mostly you'll get ingredients, but finding a decent recipe or motif is much more possible than pickpocketing.
Safebox Cracking - Probably the most fun in thieving right now, but box difficulty doesn't translate to item quality. Most master and advanced safeboxes seem pretty light or comparable to simple and intermediate. Competing with other players for safeboxes also can be problematic, but nothing severe.
Trespassing - After breaking into a house, the occupants don't sound the alarm. No bounty is incurred when they spot you. Robbing a house is low risk with decent rewards, much better than pickpocketing. Haven't encountered a master-quality locked door, but advanced doors tend to have pretty decent quality loot inside. The majority of all purple items stolen have come from advanced-quality trespassing loot.
Auto-pick - Having this function of the locksmith skill line not offer experience seems off. Players invest 4 skill points into this, only to lose out on something they use to gain. Since nothing is 100% guarantee, the fail rate makes using this function pretty useless in practice.
Achievement Hunting - A number of the achievements seem to be aimed at recklessness. It would be wonderful of some of these achievements would offer more dyes and titles. Cutpurse, footpad, thief, pickpocket, murderer, assassin, safecracker, etc. I know the introduction to the Thieves' Guild and Dark Brotherhood can offer these things, but it would be nice to see some more title options. Even for getting 1000 treasure chests (separate from Legerdemain) would be nice to get something like Treasure Hunter. Finding 1000 treasure chests is a lot of work!
Escaping Guards - I understand the need to make guards invincible, but escaping guards should be possible with invisibility potions/spells. It was right after the skill was released, but it seems that was corrected. It shouldn't be just "drink a potion and win" but it should buy you some time to move.
tl;dr - Data shows that pickpocketing is the highest risk with the least rewards. Pickpocketing, especially hard marks, should be some of the highest rewards while stealing, second to safeboxes. As it stands right now, robbing shops in lower level zones is the best way to level the skill line, and can be very lucrative at mid-levels. Safebox lock quality should match the contents. Picking a master safebox in a bank with an immortal guard hovering over you should not get me a few gold and a green drop. Trespassing should have higher risks. And don't punish the players who invest 4 skill points in locksmithing.