The second SIM slot adds an extra dimension to the Samsung Galaxy S Duos, which will only be relevant to a limited set of users, and that's what makes it a lot harder to properly judge. If you look at it as a regular smartphone, it has neither the specs nor the real-life performance the price tag implies. Dual-cores can actually be had for less, so the Galaxy S Duos will always be struggling to keep up in that race.
However, any smartphone will have a hard time competing with one hand tied behind the back. And that's the important point here - you should only consider the Samsung Galaxy S Duos if the dual-SIM functionality is essential for you and not just a nice extra that you can probably live without.
That said, the Galaxy S Duos is easily among the best dual-SIM smartphone on the global market right now. Admittedly, the bar is not set too high as tier-one manufacturers are only now starting to pay proper attention to the market segment, but even so, Samsung deserve some credit what they did.
The only problem for the Koreans is that they are not alone at the top. The HTC Desire V matches the Galaxy S Duos specs sheet in everything, but the amount of RAM and costs about the same. When things are so close though even something as small can make a huge difference.
If you don't plan on installing a 3rd party launcher, you should also consider the UI differences here - TouchWiz does have an edge over Sense UI in terms of functionality, but the cleaner look of the HTC launcher wins in terms of aesthetics.
If you have no problem going a bit further down the brand pecking order, you should definitely consider the Acer Liquid Gallant Duo. Packing a 4.3" screen of qHD resolution and a more powerful chipset than the Galaxy S Duos and Desire V, this is arguably the best spec'd dual-SIM droid currently on the market.
Acer doesn't have the experience of the two big players in the software development department, but the Liquid Gallant Duo more than makes up for that with its lower price tag. However, finding a unit in your local market might turn out to be a problem, as the Liquid Gallant Duo isn't nearly as widely available as its two competitors.
The dual-SIM market is obviously expanding to cover new ground and Samsung is not going to let itself fall behind. Its offering is, as in most other segments of the smartphone market, at the forefront of the assault. This time however the Galaxy S Duos has been quite evenly matched by the competition so it will be looking at the brand recognition to boost its market performance.
A quick look at the smartphone's specs page and its daily interest rating will tell you that this might turn out to be a far more potent weapon than you may've thought.