The program is pretty stable, having been originally made for use with higher-profile contract projects (Nuclear Throne, Rivals of Aether, Nidhogg) and still used this way. GMEdit also has an option of making a backup whenever a file is saved, making it less likely to corrupt anything than GameMaker itself. However, using version control is still a good thing for your sanity.
As for improved version, I continue to accept donations for an indefinitely large amount of hours that were already put into it, but, as we already know from Parakeet's example, it is very hard to sell GM users a better code editor - even if you put in extra months of work to have a semi-decent room editor and code analysis tools. Otherwise I'd probably take the risk of working on it for some months full-time to build a more or less complete IDE.