AMD's new Radeon R9 280X Series is based on the AMD HD 7970 GHz design, but probably comes with slightly reduced clocks to keep power draw at "saner" levels. So there is really nothing new overall, yet the card will definitely end up being a good choice for gamers because of its competitive price. In our testing, we found the MSI R9 280X GAMING, which is overclocked out of the box, to be edging past the HD 7970 GHz, achieving a slim 3% performance advantage. AMD's latest 13.11 Beta drivers include new performance optimizations for games, though, which helps in these results. Given the good memory chips used, it would have been nice to see an overclock on the memory too—our manual overclocking confirms that such would have easily been possible.
MSI's new cooler doesn't seem to be such a great implementation for this card. While it only runs pretty hot under gaming, reaching up to 87°C, which is no problem by itself, we also see very high VRM temperatures, well above 115°C. This suggests that the cooler does not cool the voltage regulation circuitry properly despite having a baseplate that should do so. Its lack of proper VRM cooling caused the card to throttle in many of our non-gaming stress tests, though gaming performance seems unaffected.
Our fan-noise test was one such test. While the card is very quiet in idle, we couldn't get the fans to spin up to the same levels under load as we did in-game. Overall, the card seems kinda noisy while gaming, but it obviously isn't as noisy as the HD 7970 GHz reference design. What also worries me a bit is the very high power consumption during multi-monitor usage and media playback. AMD could have certainly improved those two aspects with their latest generation, especially considering NVIDIA's huge lead here—the problem could be solved by picking smarter clock and voltage levels for these states.
Given a price of $300 for the reference design, I estimate the MSI R9 280X GAMING to retail for around $310, which is quite a good price. Looking at the options in this performance segment, $300 will get you a HD 7970 (non-GHz), which is slower and otherwise similar. The GHz Edition costs $375, making it more expensive for the same performance and feature set. At $300, NVIDIA has the GTX 670 with better power/heat/noise but 10% less performance. If you want to get the GTX 770, which is roughly as fast as the R9 280X, you'll have to cough up $380, a pretty large difference. So it looks like AMD's R9 280X is the best price/performance option in this segment right now, and that by quite a large margin. MSI's R9 280X GAMING does disappoint me specifically a bit though, especially with its cooler issues. Still, the card runs fine and could be a good choice if you are out to maximize what you get for your budget.

Update: After this review, MSI sent me an updated BIOS, which I posted here. It fixes the overheating issues I experienced. GPU temperature now maxes out at around 82°C and the highest VRM temperature is 101°C. Noise levels are reduced too, to around 33 dBA. This results in a huge improvement to the product as these were the major issues we experienced during the review.