It really depends on what kind of volume you expect to come through your shop. I work for a university where we have an on-campus shop that builds our desktops and it definitely has its ups and downs. One significant problem we have is in build quality we see during summer when our orders ramp up. That's when lab managers across campus plan lab replacements and the problem is that if we don't put our orders in starting in March/April they just can't build fast enough to deliver the machines in time for us to have them installed and setup for fall semester (secondary problem is that they delivered 70 machines to me in June, when I wouldn't be able to put them in place until mid-August, I had to find a place to put 70 full desktops with monitors, mice, etc). The reason the build quality slips is that the shop manager hires extra help during the summer to try and cope with the additional demand. Warranty-wise we're typically okay and we've got about 1,500 of our on-campus built machines deployed.
The other issue we have is that we often can't sit on hardware for too long. Hard drives, processors, and RAM aren't typically a big deal but we have real issues with staying on the same motherboard for more than about 6 months. Hardware gets revved, or something else stupid happens and we can't get the board we've standardized on, then it takes us about 6 weeks if we fast track our testing. Motherboard changes sound like they're not a big deal, but we've gotten boards in that we couldn't image with our imaging software, or other strange issues that are specific to the model (as opposed to a one-off bad motherboard). Also don't forget that you're going to have to start handling your own RMAs versus having Dell do it for you.
So, it's generally worked fairly well, but some of the lab managers (myself included) have sworn off any more mass orders from our on-campus shop. The main reason being that they can't keep up with the demand when we switch out our labs. We're looking at keeping 1 to 10 machine orders on campus, but anything larger than that we might divert to Dell as they have a lower failure rate on the few large orders we have done with them, and their next business day service is straight up better than anything our on-campus shop can hope to match - that's a logistics and manpower issue for them, they just can't respond as quickly to repair requests as our local Dell depot can.