Depends on your gun and what you want to do with it.
If you are new to airsoft and want upgrading have it done by a professional and buy quality parts. Don't let anyone tell you upgrading isn't worth it. You are the one using your gun, not them.
If you skirmish against opponents using quality high end upgrades to quality guns you will certainly know it! If its done properly it is definitely worth it.
There is a reason mosfets like BTC Spectre are £100+ and are always out of stock. Find some good airsoft engineers and explore your options. Also, start with a quality stock gun.
Play with hop-up, BB sizes and lipo vice conventional battery firstly... see how that goes then figure out what you want to improve.