Personally, I think Silhouette Mirage is fantastic, everything that one expects from Treasure and then some. It's closest to Gunstar Heroes in style and execution, so if you're looking for a wild, over-the-top action-shooter, you can't do any better than this. This title ranked #29 in the Sega Saturn Magazine JP readers' poll, if that counts for anything.
The visuals are absolutely gobsmacking in the their scale and variety, featuring every scaling, rotation, distortion and lighting effect in the book. You can tell that Treasure was having a lot of fun pushing out every pixel art effect they could imagine. Couple that with their trademark absurdist, surrealist humor and wild sense of color design, and you end up with arguably the finest-looking 2D videogame ever made. It's worth the ride just for the eye candy.
Gameplay is fairly simple but slightly confusing at first. The dual "silhouette" and "mirage" colors affect whether you weaken an opponent's life or spirit energy bars, and added with a large variety of attacks, give players great freedom in how they advance. If you just wish to smash through levels as quickly as possible, that can be done. If you wish to beat up opponents and steal their money (the "cash bash") to buy more power-ups, that is also possible.
Most Western gamers know Silhouette Mirage from its Playstation version, which was released in the USA by Working Designs, who took it upon themselves to make several key gameplay changes that completely ruined the whole experience, like having power-up weapons drain your spirit energy bar. The PSX translation also pares back many of the graphics and looks considerably weaker than its Saturn cousin. Oh, and the US voice acting is terrible. Ugh.
Prices for Saturn imports have dropped a lot in recent years. Where this game once sold for over $100, a quick glance at Ebay shows several copies available for under $60, a far more reasonable price. Of course, "backup" copies are found online, but if you're a true collector, you'll want to have the retail disc in your library so you can show off to family and friends.