I'm a sci-fi writer and reader, although, let's be honest - I'll read just about anything that moves me, regardless of the genre.
First, the star rating for this story is a little difficult. I'd put it at 3.5 stars. If you've already read some of the other reviews out there, you probably know one of my critiques of the story - the jump between part one and part two. Not that I minded it all that much, but it was just SO different that I had to google the plot summary of A Study in Scarlet just to make sure my e-copy had downloaded correctly.
The first part of A Study in Scarlet starts out as you think it will, in London in the 1880's and some people have been killed by what appears to be poison. Sherlock is on the case. Well, part two takes place in the Salt Lake Valley in the mid 1800's and eventually meanders back to England, but not until a seemingly unrelated story is told.
I think modern day audiences have trouble with this jump because we're used to watching modern adaptions of Doyle's work, and those adaptions DO NOT take place in 1860's the Salt Lake Valley with the polygamous Mormon settlers. This being said, I think judging the original Sherlock based off of its more modern adaptions isn't entirely fair. Doyle is the creator of these stories, and more power to him if not all facets of the story revolve around Sherlock or Watson.
(Actually I found the Mormon section of the story entirely interesting. Doyle was clearly a critic of that culture, and he was a critic during a pivotal time. Just because A Study in Scarlet is fiction, does not mean that it doesn't reflect the world events of that time. Highly interesting view point of the side effects of polygamy. I'm sure there are tons of people that disagree with this section being rooted in truth, but I'd say human behavior is not above anything I read.)
I did find the similarities between A Study in Scarlet, and the BBC's more recent adaption of the story, A Study in Pink quite fun. Having just re-watched A Study in Pink, I found myself incredibly impressed with the BBC writing team that twisted the story for modern audiences. Some of the dialogue for the show is directly from the original work, but some of it directly plays off of Doyle's work. Take the mysterious message 'Rache'. In the BBC series Sherlock is the one to dismiss Rache as being a message in German and reveals that Rache is just Rachel without the L. In Doyle's work, however, Sherlock is the one to conclude that Rache is the German word for revenge.
You may be wondering why I only give A Study in Scarlet 3.5 stars if I'm not upset about differences between section one and two. I give it 3.5 stars because I felt like the conclusion of the mystery was an afterthought. The story is fun - a mysterious message, blood from an unknown source, a cabby with unknown motives and then of course the entire Salt Lake thing - but in the end, the ending just didn't do it for me. The culprit's motivations for killing the way he did didn't make sense when compared to his life long quest to complete those killings. It seemed like Doyle had a cool idea for a mystery, but he himself didn't know how to end it, so he just patched together a semi coherent ending and left it at that.
I still think this is a worthwhile read. It's short and fun, and if you're into the BBC Sherlock, it's super fun to see how the two overlap.