Single player, by 
I didn’t play either Sonic 3 or Sonic & Knuckles, but I do know Spaztic’s excelent conversion of Marble Garden that was made years ago. But since this is single player and a new tileset conversion, I’ll rate the overall experience.
The first thing noticable about the tileset compared to Tik’s remake is the size of the graphics being x2, much like the Rayman 1 forest conversion. Other than that, it’s obviously more lacking as it doesn’t contain JJ2-like features such as poles, vines, tubes, destructable blocks, etc.
The levels are generally OK, especially for a first upload. Both levels make a big deal out of diagonal ramps, a feature that doesn’t work too well in JJ2 as you can’t run down from them without falling forward. Yet on paths going upward or forward I genuinely got a Sonic-vibe that kept me running without too much hassle from enemies or obstacles. Eyecandy is very little, all in the form of those plant-pillar things. The background is non-textured blue with moving clouds.
In terms of pickups, there are enough carrots scattered around, coins for the usual coin warps and savepoints. Ammo is restricted to Bouncers and the Toaster, which might be a bit lacking even though they can be upgraded (with PU’s) at some point. Enemies consist especially of Lizards and Fencers, but a commonly used hazard in these levels comes in the form of the Rolling Rock. It’s used in pretty much the same way as in the original JJ2 Jungle level, until at one point they’re used as still platforms, giving players the fear that they may fall down at some point by touching a Trigger Rock.
Another fun thing to point out is the bossfight. Something as simple as placing Bubba on rows of Collapsing Scenery is still pretty fun and works well to make the boss more interesting.
All said and done, I recommend trying this download only if you enjoy Sonic games or conversions in general. If you’re looking for a Marble Garden tileset you’re better off with Spaztic’s conversion. For better Sonic themed levels I recommend, of course, Sonic With a Gun! I’d give this 6.5 for the OK conversion and gameplay, but slightly raised it to 6.7 as it’s a solid job for a first upload! :D