I'm not going to lie, I think my wall turned out pretty damn good. The teal looks great with our bedding and the hardwood floors. The contrast from the teal diamonds and white stripes looks sharp too. Seeing that I had nothing better to do with my 12 hours (not like I really want to go to the gym anyway), I think it was worth it.
If you think this is a project you want to take on, go for it! But first, heed my advice:
1) You definitely need a ton of patience, but you also need to be detail-oriented. If you don't care if your measurements are off a bit, your lines are crooked or the paint bleeds, you aren't going to end with a perfect wall. I spent a lot of time taping and re-taping because my measurements were off and my lines looked crooked. I also spent a good chunk of time touching up the white lines after the teal bled. If you rush this part, you're going to make an even bigger mess.
2) Working on this alone made a slow go of it, but at least my husband and I didn't argue for 12 hours straight. If you can work patiently and productively with someone else, I would strongly recommend it. I think my measurements would have been better if I had someone holding the tape measure. Although now looking back, I should have just taped the tape measure to the wall to keep it consistent.
3) My husband had suggested to use a laser level, like this one from Black & Decker for $45. It probably would have been a great investment, but I'm cheap and didn't want to invest in one. If you're going at this alone, I could see a laser level come in handy.
4) Get an artist paintbrush for touch ups. I've done a few paintings in my day but could not find my little paint brushes (go figure). Having one of those would have made my touch up process a lot faster. Instead, I just used my normal sized paint brush, which required a very steady hand.
5) Paint bigger diamonds. I made my diamonds 24x12 inches. While I think it looks great, I think 30x15 diamonds would have been better.