well, it really depends on what you want. If you want to fly only multirotors then the stock firmware is probably ok..
If you want later to fly the specktrum BNF planes that use a different channel order for example, you will have to upgrade to a custom FW, as it is then possible to assign any input to any channel..
If you need to do more complicated mixes, have access to telemetry (many people have frsky telemetry), audio, etc, there are a whole lot of modifications you can do.
If you're not good with the soldering iron, there are other options.
For flashing the custom fw you could use the smartiepaRts board which is a solderless option to flash the radio..
For a solderless telemetry option, you have the TelemetrEZ board.
For voice there is also a voice module you can buy although is not completely solderless.
Of course if you're good soldering small components and like to play with electronics, you can build everything yourself. the wiki explains how to do most of them. for other tutorials search the forums or ask.
As far as the firmwares are concerned, they are both excellent.. Just try both.. er9x is probably easier to start with as it is a little bit more straight forward to download for example..
Open9x is also an excellent firmware and it is a bit more elaborate, with more features and more options.. The only downside is that it doesn't fit completely in the stock board, unless you upgrade it by changing the processor. So you must choose some options, and leave some out when you download the firmware prior to flash it to the radio..
With er9x you don't have this problem.
You can also download eepe for er9x and companion9x for open9x.
Just install them on your computer and play with them. they simulate the 2 firmwares and can also flash the radio and copy models and settings from/to the radio. It's better to use them to learn to program.
Ifyou decide to give it a go, please start with an open mind. these work quite differently from the other systems we were used to before. it's a learning curve and it can be quite steep at times. but when you start to understand it you will have a lot of "Hurray" moments
Just give them a go. with the computer programs you don't even need to flassh your radio to learn how to work with them.
About the stock RF system.. I didn't fly mine for very long as I moved to frsky with telemetry. But I don't remember having any problem with it. some others had. I would of course recommend you to move to frsky. it's not that expensive, it's good, and even if you don't want on screen telemetry, it will warn you if there is a range problem. and only for that it is already worth to get the system..
Oh, I can see it is your first post, so
BE VERY WELCOME TO THE 9XFORUMS
And as I always say, I hope you like beer
João