I arrived in Devin by bus from Plovdiv. The journey takes about 2,5 hrs and the ticket costs 7 BGN. Another option is a to take a direct bus from Sofia (supposedly there's one per day) or to take a bus from Smolyan.
Be aware that there isn't hardly any connection on Saturdays.
The only public transport available for getting to Devin is the bus transport. If you can choose between taking the bus and going there by car I would strongly advise you to choose the car. As you can see from the picture most of the busses going that direction are old and sometimes during winter season it could be quite cold inside.
There are regular bus lines to Pazardzhik, Plovdiv, Krichim, Smolyan, Dospat, Gotse Delchev, Shiroka Luka, Pamporovo and other smaller villages within the region. The bus station is at the very entrance of the town, on the right bank of the river.
As far as I know there are buses for Sofia once or twice a week but you'd better call the bus station for more information.
Bus station address:
24, Osvobozhdenie Str.