Depending on your use case, this controller will either be really good or pretty mediocre with no in-between. This review will be from the use case of indie PC titles and some modern games, with some original Saturn games as well. I`ve bought 2 of these controllers and have had issues with both of them. That will be a separate review for the purposes of readability. tl;dr at the end
The Saturn Pro controller is modeled off of the original Japanese Saturn controller with some differences, namely the additional shoulder buttons, Home and Select buttons, and Hall Effect analog sticks. These, along with the added internals change the way the controller feels in the hand in ways that help it feel less like a toy. The added weight from the battery gives the controller a much needed heft. The L and R buttons are flat instead of having a unique shape and the resting place for your fingers is now on the ZL and ZR buttons, which are raised on the ends to keep your fingers from rolling off the back and make every press feel deliberate. The island in the center where the miniature analogs sit change the curvature at the bottom to look more natural and make the ends less wing-like but otherwise still rest pleasantly anywhere in the hand.
When playing a game like Monster Hunter Wilds two things immediately become apparent. One, the analog sticks are too close together. Because of the size and placement of the joysticks my average length thumbs have to stretch to reach the analog sticks which gets really uncomfortable for long play sessions but you do get used to it over time and the fatigue goes away. If you have longer thumbs, your thumbs end up bumping into each other due to the short range of motion the joysticks have. The second thing that becomes immediately noticeable is the mapping of the Z and C buttons on the controller. Retro-Bit has stated in replies on media that the Z and C buttons are hard mapped to R and L respectively. If you`re using those buttons on an replicator, you end up using ZL and ZR as your L/R buttons, and L/R for your Z/C buttons. You don`t have to worry about this if you`re just using the ABXY buttons, however.
I played through the entirety of Monster Hunter Wilds, and the only issue I had with the controller was the constant disconnecting and reconnecting the controller did. By default the controller only connects wirelessly via 2.4GHz dongle either to your Saturn or PC depending on which dongle you`re using. The controller would immediately reconnect after disconnecting, but sometimes I would be sitting for a good 30-40 seconds waiting for these disconnecting fits to finish before I can get back to the game, and in an action game is a devastating amount of time. Only the first controller had this issue so your mileage may vary, but there is also now a firmware update to add wired mode and that wasn`t around until recently.
If you`re using this controller to play sidescrollers and retro games, it`s nothing short of fantastic. The D-pad is famous for being one of the best and that is still the case here. I have played through 20XX, ~zero in on Holoearth~, BLADECHIMERA, a bunch of Touhou on the PC, and the Mega Man Zero/ZX Collection on the Switch with this collection, and this is all to say if you`re playing retro or retro style games, this is an amazing controller. The extra buttons are welcome for shmups, the D-pad never dropped an input, it feels great to hold and the battery lasts forever. If you`re playing modern games, it`s an alright Xbox controller but you could do a lot better.
Very brief time was spent using this on the MiSTer and Sega Saturn so there`s not a lot to say. The MiSTer accepted it no problem, I played a bunch of Spyro with it and it worked as expected. The Saturn, however, is where this falls apart. ZL and ZR now serve as the in-game L and R buttons, while the actual L/R buttons serve as C and Z, as noted in an earlier paragraph. I didn`t play Nights to test analog mode, but the shoulder buttons are digital instead of analog so if you`re planning on using this as a replacement for the official Saturn Analog Pad, just get that instead. If you`re only interested in using this as a wireless Saturn controller, Retro-Bit has other options that probably work better for this specific purpose.
tl;dr: Feels slightly different from an original Sega Saturn controller, still excellent for retro games and retro style games. An okay controller for modern games, kind of sucks for the Sega Saturn. Hardware issues hold this controller back.
