While the  pizza we ordered from the Forty Niner wasn't terrible (it was ok, if you don't mind sauce straight out of the can) the ambiance was terrible.  We went in for lunch on a warm July afternoon and the Pizzeria was sweltering.
My sister and her hubby are vegetarians so we ordered a pizza with olives.  They were out of olives.  Really?  There was a grocery store two doors over, and according to the pizza maker, it  and the tavern in between were all owned by the same person.  Each venue was open to the next so you didn't even have to go outside to travel between them.  Hmmm, wouldn't you just walk the 30 steps to grab a can of olives?  You wouldn't if you knew what the market looked like!  See my review of Bigley's if you want a fuller description of the "market"'. (We became acquainted with Bigley's when the pizza lady told us we could either buy our drinks from Bigley's or the Brass Rail Tavern.)
The seating area for the 49er was shared with the neighboring Brass Rail Tavern, which was a bit cooler. Â When we went into the tavern side we were confronted with grimy, sticky tables. Â Ok, no big, I've eaten in plenty of grimy places before- so what, it was a dive. Â It was the middle of the day and an aging "girl" in a short, sequined dress was playing pool by herself. Â A poster by the bar announced that on the 4th of July a tattoo artist would be in residence the whole day for those desiring a new tattoo. Â I admit that I don't have any tatts, but if I was going to get one, I'd rather get it in a prison cell than in this dump. And at the bar a sad collection of drunks and druggies.
So, here we are, middle of the day eating our rather dull pizza in the almost empty  saloon and one of the drunks at the bar decides to stagger over and harass us.  He wanted a slice of our pizza and I told him no.  He continued to pester us and offered me $100 for a slice.  I told him he could buy his own pizza for less but he continued to insist we give him a slice.  Dang, it's a Tuesday afternoon and the bar is practically empty and the bartender won't reign in the drunk ruining our lunch?  Finally when I said "stop it" in a tone of voice I typically reserve for a yapping dog, he stumbled away.  But lo and behold, a few minutes later he returned with a buddy to continue to bother us for a slice of pizza.  Really?  If I didn't know better, I woulda thought he was trying to pick a fight with my peace loving, mellow hippie bro-in-law or my gentle but strong bf.  I was the only one with a fighting temperament, so my peace loving, mellow hippie sister begged him to leave the table.  Finally we left just to escape him and his ilk.
UPD01JUN2013: It looks like this place has closed? I'll update this when/if they re-open.
25JUN2012:
This small restaurant  is in the same building as the Brass Rail Tavern in North San Juan.  The staff is always friendly and the decor is... interesting.  I'm giving Forty Niner Pizzaria 3/5 stars because I love their "Murder Burger" with the spiced ground beef with everything on it: BIG TIME YUMMY!  Be advised: you specify "medium" or "medium rare" to avoid over cooking!  The order comes with fries or onion rings and we're talking generous helpings here.  The price is very reasonable and is a good value.  With the exception of Lefty's Grill in Nevada City, these folks make a better hamburger then any place I've recently eaten in either Grass Valley or Nevada City.  Parking is across the street on a dirt lot or along south bound highway 49 in front of the building.  You can call ahead for pickup, too.