I hear this all the time and it drives me nuts - many people believe that when you buy a custom bike, you're buying *really good welding*.
Folks, I would bet money that most of the welders making cheap Raleighs in Asia are better than me. In fact, I bet most of them are better than every single custom framebuilder out there. Why? Well, it's simple - those guys don't answer the phone. They don't design the bikes. They don't miter tubes, set up fixtures, paint stuff, ream/face/chase threads and press fits, or ship things. They just sit in one place and weld. I'm pretty sure you get DARN good at welding bikes together after doing it as your 9 to 5 (or, well, who knows how long those guys work) job for a while.
Are welds on overseas frames as pretty as on a custom bike? That's really in the eye of the beholder. Are they as strong? Obviously yes, since like custom bikes, very few mass produced bikes fall apart and almost none do so because the welds weren't done well (poor design is a whole different can of worms).
So here's the thing - you aren't paying for welding when you buy a custom bike. You're paying for custom fit and design first and foremost, and then things like customer service, warranty, cachet (in some cases), and getting to pick out things like parts and colors. If you pick any decent builder, the welding will be great - but it won't add anything to your experience once you hit the trail. A great fit and the right parts, on the other hand, will put a grin on your face every time.