Ah, the ever-present question!
It really depends on your own needs. If you just want an easy-to-grab player for music-listening and some recording, with a “typical”-sized music collection, and an FM radio, the original firmware likely will be just fine–it really works very well and is easy to use. You might want to start out there, get used to the player, and see if you want anything more.
Rockbox also is great (I don’t have it on my Clips, yet, but almost certainly will soon, and have had it on an iRiver player for 7-8 years). Lots of options, which some people like but which can overwhelm others–reading or at least skimming the manual definitely is recommended, to verify operation and see what’s available. Instant nice features: a customizable screen with many user-developed screens available, and a better database that will take any size of music collection that you can thrown at it (the original Clip firmware can start getting buggy and peter out between 5,000-8,000 files, and its “database refresh” anytime you add or take away files can take 15-20 minutes or more, depending on the size of your colletion). Note that if you install Rockbox, you aways cen fall back to the original firmware when you start the player. Importantly, also note, if you install Rockbox, that you may permanently disable the ability to play DRM-protected files on the player, even if you later remove Rockbox–important to consider if you may use Audible or library-protected files on the player.
In the end, both firmwares are great, with Rockbox almost like an “advanced” firmware for the player; you may want to peruse the Rockbox manual (individualized per player) at rockbox.org–recommended. Aren’t choices great?