I have been using my 150Ah DIY lithium (with 120 cylindrical cells) for going on 4 years now and it has worked wonderfully. If I were to do it again, I would use the 120Ah prismatic cells that are currently available at very competitive prices. These were not cost competitive 4 years ago.
Building the battery is very straightforward - but you need to respect the energy that is contained in the batteries. There are a few caveats:
1. You want to have a way to 'top balance' the batteries you purchase before you install them. This is usually done by connecting them all in parallel and charging them to 3.5V with an adjustable power supply. You may be able to get away without doing this, but omitting this seems to be a leading cause of future problems.
2. You want to clamp the batteries together, to prevent them swelling.
3. You should place some sort of insulator (eg 1/8" plastic sheet) between the cells. The metal cases are also the battery negative, so if you were to damage the blue heat shrink wrap, you risk shorting out the batteries and a fire. You should also cover the exposed terminals on the cells.
4. You need to set up your BMS correctly.