Reasons I trust them:
1. You won't get ripped off, gives somewhat exact quotation beforehand, no surprises later.
2. The owner himself looks over the job, if it's massive, passionate about it.
3. I believe, they give each job a personal touch, which, again in my belief, an important factor, if you really love your bike.
4. A free sticker to remind you of the next service (it's not sticked at an easily visible part, won't spoil the look of the bike)