While Temecula is not Napa, it does have some up and coming and promising wineries. We have found a couple of very solid quality boutique vineyards on the east end of Temecula.
Palumbo Family Vineyards - 4 excellent offerings, 3 excellent reds and 1 white. Small family tasting room, nice property, and knowledgable staff.
Doffo - Another relatively small family operation. East of Palumbo specializes in very bold red wines, most wines are estate grown and the tasting room is lively. Their wine quite good, in my opinion, and many of their wines are built to age.
Weins - A much larger family operation but has solid selection of reds and nice selection of white wines. A bit more commercial than the two above but have always had a great experience here. Their wines are approachable on both taste and price.
Those are my favs so far. There are about a dozen or more vineyards in the area but based on my research these 3 seem to be doing solid quality.