Before the review, I'd like to say I'm a fan of Wario Land games. I've played them from the very first Wario Land to Wario Land Shake It on the Wii. It's funny that I've never played this one because I never had a Virtual Boy.. Until now! It's a solid entry to the Wario Land series and it's pretty great!
(Note: I use "VB" as the abbreviation for Virtual Boy. This is to save space for the review.)

--Graphics--
I wanted to save the graphics section for last, but I think for the sake of everyone's retinas, it's a game for the VB. It will be in red and black all the time and it might hurt your eyes after a long period of time. So for your own safety, don't play this for hours! Take frequent breaks to get a good experience with this game.

Now that the safety precautions are out of the way, let's talk about the graphics themselves. The graphics are pretty well done and fit into the style of how Wario Land is. Animation-wise, everything is fluid. It's "easy" on the eyes. (Not the best way of putting it, but should be easy enough to understand.)
Though the style of graphics are pretty good, they have demonstrated some neat tricks with VB's technology as well. Example: Wario uses a jump pad to jump into the background and then uses another jump pad to jump back to another part of the stage. I found that pretty cool for the VB's technology.
Unfortunately, like all of the VB games, it's always in black and red. It's the only fault of the game in my opinion.

I give the graphics a solid 9 for good use of the VB's capabilities and great graphical style.

--Sound/Music--
Sound-wise, it's like a more advanced form of what the Game Boy's audio was. It's definitely listenable and fits the style of the music from Wario Land. The music can be pleasant at times and that can be enough to enjoy it all the way through. As for the sound effects, they're more geared to the early Wario Land games in terms of quality.

I give the sound and music an 8. You're not going to remember this game for its audio, unfortunately. This does get some bonus points for making the music fit some parts of some stages, though.

--Addictiveness / Depth--
Like the older Wario Land games, it's pretty fun to play! Plenty of powerups to fool around with, such as a Bull Powerup that has an earthquake-like ground pound and a Dragon Powerup that shoots out flames! (It works underwater, which is a plus.) It also has a good selection of stages and some bosses, so replayability is a plus with this one.
As for the amount of content.. Every stage requires you to find a key at the end of every stage in order to advance, which can take some time and effort. At the end of every stage, there are two minigames that you can choose to do. One minigame can increase your hearts (Getting 100 hearts equals a 1UP.) and/or multiplying your coins with a coin minigame. Coins total up to your overall coin total, so if that's your sort of thing, try to get a huge coin total before the game ends. (It's possible to get 9999 coins in the coin minigame!)

Depth gets a 9, Addictiveness gets an 8. The addition of two minigames in between stages increases the addictiveness, but that's about it other than the game itself. The stages themselves might be fun enough to play through for attempting a high score.

--Overall--
Overall, VB Wario Land is a fun game that's worth a play. Though it's "unfortunately" on the VB, don't let that stop you from playing it. If you love Wario games in general, this is still worth a play.

I give VB Wario Land an 8.6 out of 10. VB Wario Land definitely one of the better games on the system and I recommend this to anyone who enjoys Wario and/or platforming. It's also a great start for those who want to get into the Virtual Boy collection.