Well, I finally broke down my 4x8 sheet of Plexiglas G and built my first sump. I have built other acrylic projects before (a couple of decent sized protein skimmers mainly and a few other small jobs), but this was the first major project I did that used the pins method before. A few observations:
1) The first joints were definitely the best. This was when I was attaching the ends and dividers to the front, one by one. Each of these came out pretty much perfect! No bubbles at all. I am VERY pleased with each of these joints. I used a pair of 12" plastic speed squares to align the dividers. I used carpet tape to attach the speed squares to the right spot to hold each divider in the perfect alignment as the solvent did it's work. That worked out VERY well and really took a lot of concern out of the gluing process.
2) When I attached the second side, I obviously had to glue all the dividers at once. These joints were ALMOST as good as the individual joints, but a few of the joints on my bubble trap ended up getting messed up slightly because I had to shift them around slightly after the solvent had been added. I originally had them in the right place, but they shifted on my slightly. I did not account for just how flexible a flat sheet of acrylic is (the front in this case), so they were able to move more than I thought they would. The movement caused some bubbles in the center bubble trap divider, but the joint is water tight and very secure. Just not perfect.
3) Attaching the bottom was a bit scary. It was being glued to both sides, both ends, two of the three bubble trap dividers and to one additional divider. All at the same time. There were a couple of areas were I had some bubbles in the joint one area were I thought I needed to go ahead and put a bead of #16 down later to make SURE that no water could work it's way around a close spaced series of bubbles and cause a very slow leak.
4) When I attached the top, I FINALLY figured out what I had done wrong on the bottom: I had the pins spaced too far apart. This allowed the acrylic to push together ever so slightly and prevented sufficient solvent from getting into the joint. Of course, I figured this out when I was about half way through applying solvent to the joint, so I couldn't stop and add more pins! Needless to say, I ended up with a couple of places with some bubbles in the acrylic. The top is on VERY securely and I really don't think I will have any leaks, even if I DID run the water level all the way to the very top! But it doesn't loop perfect all the way around, so it irritates me. At least 95% of the joint IS perfectly clear with no bubbles, so I can take some solace in that!
5) I used a 1/8" solid carbide spiral up-cut bit to remove the bulk of the waste from the opening in the top before I went back with pattern bit and trimmed the top to perfection. Using the 1/8" bit to remove most of the waste increased the size of the waste piece by over 1" in each direction AND it helped keep the amount of acrylic "flakes" (aka: dust) down in my shop because I did not machine away nearly as much acrylic as I would have.
All in all, I am willing to call this pretty much a 100% success. I'll never be COMPLETELY happy until each and every joint is absolutely perfect, but 99% of the people that look at it now will never see the bubbles. Until I point them out!
Thanks for all the great ideas and suggestions in this thread and some of the other acrylic threads. I KNOW I would not have done nearly as good a job with out that advise.
Oh, on last thing. I have seen some comments on reusing the acupuncture needles. In my case, I have already reused these pins several times. After I use them, I let the acrylic on them set up completely and then pinch the needle between the nail on my thumb and fore finger and then pull the needle through my nails. It quickly and cleanly strips off any remaining acrylic and other waste and leaves the needle ready for the next use. I stuff them back into their plastic tubes and store them later.
And I've also had a lit candle nearby just to unclog it as it occurs. 
