Its one of the better 3D Sonics on the market.
The game has issues with its level design towards the end and its controls can be hit and miss to some people (Ex: People either love or hate the run button).
But from what I saw of the game, its a good game

.
The visuals look great and pop, the music is great like most Sonic OST's, the core gameplay is a nice mix of SA1 and Sonic Unleashed, the levels look like they were ripped from the Genesis era and a good deal of the level designs are strong.
Its just that after Frozen Factory, the quality starts to drop and as a result, people either start hating the game or feel underwhelmed.
Lava Mountain is a case of a really weak final world for example: one boss rush, a harder version of Tropical Coast Act 3, another boss rush mixed with a platforming level, then a weak final boss (ripped right out of Sonic Colors). That leaves a bad impression but the game as a whole is good.
The game isn't great like Colors or Generations but its still a solid game. In many ways, its like Unleashed if you didn't have the Werehog; you have a game that experiments and tries new things frequently but the games core is still present, not hindered with unneeded gameplay changes. You still get those gimmick levels, but they take up one/two spots for each level.
Overall, Lost World is a good game that has a few issues but due to being a step down from Generations and Colors, people have a harsher tone toward the game.