The Dunlop 202 is a glass bottleneck slide that is designed for use with electric and acoustic guitars.  This is probably my favorite glass slide that Dunlop has to offer as it gets the job done in all ways.  It has a great sound and is definitely a comfortable fit granted that you get the size that will work best for you.  It is made of a sturdy material that would be pretty hard to break even if you wanted to.  I can't say that I have any complaints about this slide at all as I use it all the time with both my Martin acoustic guitar and my Fender Strat electric with great results.  While I would recommend using this for normal dreadnought acoustic guitars, I would probably recommend going with a metal slide if you are going to be playing in an open tuning or with a dobro style guitar as this is my personal preference in terms of tone as the glass and the metal slides definitely have their own distinct sounds.  I personally have a number of slides including this and a metal one that I will use in different situations as they can both evoke different sounding tones.  However, I would probably recommend going with a glass slide like this if you are getting your first slide as to me they are the better sounding overall of the two.  The Dunlop 202 is overall a very well made slide from a company that definitely has experience and knows exactly what they are doing.  The price very cheap and even if you don't end up liking this style of slide, it isn't a big loss at all because of this cheap price.  I definitely vouch for the feel and sound of the Dunlop 202 bottleneck slide.