I have noticed that with companies like Groupon here in Spain there are some good offers, but there are also a lot of offers that are not truly discounted. For instance hotels are rarely truly discounted. The 56% off advertised is only in relation to the maximum stated price, which is only ever employed on a holiday weekend during the high season. When a hotel offers a 56% discount in March, that “discount” is actually their normal non-discounted rate. You can get the same price or better at a site like tripadvisor or expedia.
The same is true with most restaurants, where the half off price is often more than what other similar restaurants are charging for their normal prices. Obviously, the Groupon half price is based on some mythical full price.
Concerning electronic items many are not even discounted, and when they are the discount is based on an initial MSRP, not the current street price. I often find that the Groupon discounted price is equal or higher than the street price.
There are still some very good deals in sites like Groupon, but you have to look around, and they tend to be in the minority. Look carefully, don´t be fooled and only buy stuff on Groupon if it really seems like a genuinely good deal.