I am doing a project with around 100 parts, mostly SMT , and some thru-hole parts. So I will try to get someone to assemble this board with parts installed.

The question is, who? ... So far [advertising removed] seems to offer this service, and I got a rough quote two days ago suggesting something like $290.

Anyone have experience with this kind of thing; with bittele, or anyone else?

I must say that all of my experience has been with soldering up prototypes myself. I have used DorkBot for my PCBs, and then just soldered the SMT and through-hole components myself. They're quoting you $290 for a one-off prototype? Out of curiosity, what kind of components are we talking? Any BGA, or purely edge-soldered components?

I go for through-hole parts whenever possible for prototypes for precisely this reason. If you have access to a reasonably well-equipped lab it'll take some time to assemble, but shouldn't be too hard. I've done mods involving SMD components (swap out a few resistors, etc) without SMD tools, but I wouldn't want to do a whole assembly without paste and hot air/reflow.

I'd ask around and see if you can get access to some decent tools. Failing that, $300 isn't too insane for a one-off with short turnaround.

this will be my first complex design with SMT chips (BGA, LFCSP, LCC, CSPBGA, etc.) that I can't solder by hand with the tools I have
Hand soldering ( e.g., BGA reflow in a toaster oven) is not an option for me right now; neither is investing in expensive equipment.

turnkey pcb assembly

Then it sounds as if this would be your best choice. If $300 isn't prohibitive for your budget (what is this, personal project, class project, work prototype?) then it sounds reasonable to me. Will you be doing the board design yourself or letting them do it? I don't know if I'd trust them to get all of the component trace specs correct.