Is it worth to purchase Premium?
Beat the game. After you do, if you want to keep playing, buy premium.
The best reason I can offer to buy premium is if you really like the developer, appreciate his previous games and want to encourage him to continue making more. As far as the game itself goes, premium is totally unnecessary. Labyrinth is by far the easiest game in the series, and the bonus skills just aren’t very important.
If you want to pump up your wizard level to uselessly high levels, premium is pretty much essential. Like almos up there, who’s at wizard level 6987. For comparison, I beat the game without premium at wizard level 86. Not only does premium give you unlimited waves on any map you want, premium also gives you access to basic settings that can give you roughly 4 times as much score on every single map. A player without premium will never have as high a wizard level, or as high scores as a player with it. But neither are very important for regular gameplay: If you play without premium you probably won’t ever miss out on the extra levels. There are also several journey and challenge amulets that are impossible to get without premium, so if you want to be a completionist and “do everything” then you’ll need premium.
If you really want to enjoy the endgame, I personally enjoyed Chapter Zero much more after beating it than I ever enjoyed Labyrinth at any point, before or after. Basically…yes, premium does offer useful tools and gives you something to do after beating the game, but Labyrinth is the weakest game of the series for play at that level. So if you want to play at that level, why are you playing Labyrinth?
One last thing to consider: personally I played through without premium, and even through most of the second half I felt like I was cruising through so effortlessly to the point of boredom. Once you learn a couple basic things the game becomes so easy that it becomes very dull. With premium, I would expect a player to probably reach much higher wizard levels much sooner, and thus trivialize the game even more. There are probably a lot of middle-game levels that can be won simply by placing a wild gem in a tower on the first wave and then walking away from the keyboard.
not a frequent player
Premium mostly shines after you beat the game. If you’re not going to stick with it long enough to beat it, or if you’re not going to keep playing after you beat it, then premium probably isn’t for you.