I've got one, got it on special sale for about that much a couple of years ago. It's decent for what it is, but I find it better as an instrument mic compared to a vocal mic. For vocals, it's a tad underwhelming in the lower mids and the "focus" control is more of a gimmick than actually useful in my opinion. I've used it as a room mic for drums, on hi-hat, and as secondary mic on guitar cabs with good results. If you're looking for something to use on vocals specifically, there's probably something better out there for a little more $$, but considering that for that price you get a decent shock-mount and pop filter (mind also came with a good quality cable, not sure if they're still including that or not), it's definitely a good bang-for-the-buck utility mic.
I've been using a Blue Spark for about 5 years now and absolutely love it for many applications. The quality of sound for the price is really hard to beat.
Forgive the incidental self promotion, but here is a song I recently released which heavily features Blue Spark tracks. I saw you want to use it for acoustic - the acoustic guitars, vocals, and a drum overhead were all done with the Spark into a Presonus 16.0.2. The main guitar line was double tracked, each using the Spark, with a third "ghost" track on a Fender Parlor guitar hanging in the background, which was also done with the Spark.
Hope thats helpful! The guitar was the same Martin Koa pictured in the video. Blue Spark was placed about 8 inches away from the upper frets near, but not directly over the sound hole.