03.04.2018 : All operators info added.
07.09.2017 : Blood Orchid Operators info added.
12.07.2017 Edit : Added gain info for ranked & casual or terrorist hunt.
Added how many R6 Credits enough to open operators info.

Is Rainbow Six Siege STARTER edition worth buying ?

Yes ,
I have played 11 hours and now I can open 1 operator. You can open 1 character between 12 and 17 hours ( average) . Normally 500-2000 in game point enough to open operator, At Starter Edition this is required 12500 point. (usually you get between 100 and 200) (You can also earn from Challenge in Uplay) However, Starter Edition gives you 600 R6 Credit bonus (You can buy with 300 - 600 R6 credits open to operators) , So you can open up to 2 characters immediately. Again, Starter Edition specifically gives you 2 operators at the beginning. (Two from of Rook, Sledge, Ash, Fuze, Mute, and Smoke).
In summary, it is very difficult to get learn to the game with 12 hours, so you have to play at least 30 hours for you will be good at this game .Thus you can open up to 6 characters with 600 R6 credits. ( At first 2 characters, with 600 R6 Credit buy 2 operators and 30 hours is equal 2 operators )
It is very difficult to get out of the game, you said to yourself '' i will play 30 min ''at the beginning, but you dont exit the game, it was a very nice and interesting game.
This game is slower than CS:GO, Bf or COD. Team play is very important, communication is vitality . There are many ways to reach the goal. ( Breach the doors, windows, equipment, close combat etc.
In 12 hours, I could kill only 15 people in total. I need to play thoroughly in Terrorist Hunt and Situation. I've never seen low-level people, the only negative side in my opinion.
Have a good days, Sorry for my bad english :)

21.03.2017 Edit : After 35 hours gameplay, unlocked 7 character. You should follow discount operator or bundles. In addition, you can unlock 24 hours Renown Boost pack ( 1 piece ) by Uplay Points. You will get %100 more much gain Renown after match for 24 hours. For Uplay points , you must unlocked game actions.
03.04.2017 Edit : 8 operators unlocked right now with in 52 Hours.
13.04.2017 Edit : 10 operators unlocked right now with in 78 Hours.
12.07.2017 Edit : Finally all unlocked.