My out look would be this.
If your A/C will not function and is empty. You need to go to the shop.
If your A/C cycles, or just isn't running cold or takes longer to get cold. Then a DIY can be done.
Here is my thoughts on adding liquid to the low side in a cycling/working but low system. There is hot gas in the line being cycle. The thought is, if you add the liquid to the gas, it, itself should turn into gas and expand in the lowside as soon as it enters the system. Therefor bring the psi up in the low side of the A/C system, and never reaching the compressor in a liquid state.
