The d20 Modern system attempted to, in some way, make the traditional six-attribute D&D spread into something worth talking about - something no edition of D&D has ever done (nor anything any edition of D&D ever will do). The class feature and feat systems always melded with a great deal of uncertainty - what made something a good feat and what should be relegated to the class features. And, despite trying, without significant support for nonmagical ranged combat in the D&D system underlying d20 (something that's persisted to the present day - just try playing a nonmagical archer in 5e! Just kidding, don't under any circumstances do this), gunplay never seemed adequately exciting as combat. The only application that worked for it, Spycraft, essentially built its combat from the ground up on gunplay.
Sidewinder: Recoiled is probably the best version of a d20 Modern campaign supplement. Within the lines of d20 Modern, they created a very straightforward adaptation for a "modern" (i.e. non-fantastic) setting, adapted the class features to the setting in ways that express what a "smart" Western hero is, and did their best to make the feats, the building blocks of the D&D3-era character, interesting and evocative.
That said, Sidewinder: Recoiled should be considered a "generic" Western d20 Modern supplement. It doesn't go into great detail on how to navigate the thorny historical waters of race, gender and politics; it doesn't help with the conception of a campaign or collaboration on characters to have an exciting Western adventure. All it does, though this is not a small accomplishment, is take the action-adventure engine of d20 Modern and say "okay, here's a Western version of this." It assumes that you know what you want to do in a Western action-adventure, that team-based gunplay is the primary way you want to interact mechanically with the world, and (perhaps most artificially) that advancement, in this world, corresponds to the advancement of combat power.
It's a strange thought, the "advancement" of combat power in a Western, when most actual Western films portray the most deadly fighters as youthful and impetuous. But perhaps the idea in Western RPGs is that you're looking at narrative deadliness and not necessarily an inclination to violence. On the other other hand, isn't the inclination to violence, at least in part, a function of whether you think you'll be badly hurt by it? A little too deep a dive for Sidewinder: Recoiled.
Nevertheless, for the GM who wants to use a system familiar to players of D&D3 or its successors (Pathfinder, D&D3.5, etc.) to play a Western action-adventure campaign that they already have in mind, Sidewinder: Recoiled is a very solid outing. It is perhaps the best case for the "universal" D&D3-era approach to d20 gaming, which aimed to create a single system which could be used for multiple campaigns or concepts. However, by the same token, it doesn't expand much on the D20 Modern ethos, and gives very little concrete advice for dealing with the challenges of a Western story or identifying how violence and action is used differently in that story than in D&D. (Perhaps tellingly, there's a whole chapter on animals combat stats) I'm giving it a solid score because it really does what it sets out to do; but perhaps there's a reason the big d20-era games we remember today are ones that took big swings at altering the system deeply (Mutants & Masterminds, Spycraft, Blue Rose) while games that just did what the "theory" of d20 suggested, like Sidewinder, left us with that system.