It depends on what you expected out of a 5870. Here is how the 8800gtx, 9800gtx, gtx280, 4870 and 5870 performed when they were released:
http://i34.tinypic.com/2e499ua.jpg
http://i35.tinypic.com/v2zaxv.jpg
Not as impressive as a 8800GTX or HD4870, however miles farther from a 9800GTX, and even better in comparison to the GTX280.
It's impact in performance over previous gen is right between the impact made by a GTX280 over a 9800GTX and 8800GTX over a 7900GTX
LoL come on. You know as well as anyone that the 9xxx series were not a new generation. And also you should compare the GTX280 with the 8800 GT better than 9800GTX, because specs wise it's closer to half a GTX280 than the 9800GTX.
GTX280 => 32 ROP, 80 TMU, 240 SP, 602 Mhz core, 1296 Mhz shaders. Most Importantly: 19GPixels/s, 48 Gtexels/s, 933 Gflops, 140 GB/s
1/2 GTX280 =>
16 ROP, 40 TMU, 120 SP, 602 Mhz core, 1296 Mhz shaders, Most Importantly: 9.5 GPixels/s, 24 Gtexels/s, 466 Gflops, 70 GB/s.
9800 GTX =>
16 ROP,
64 TMU,
128 SP,
738 Mhz core,
1836 Mhz shaders, Most Importantly:
11.8 GPixels/s,
47 Gtexels/s,
705 Gflops,
70 GB/s.
8800 GT =>
16 ROP,
56 TMU,
112 SP,
600 Mhz core,
1500 Mhz shaders., Most Importantly:
9.6 GPixels/s,
33.6 Gtexels/s,
504 Gflops,
57.6 GB/s.
1/2 the GTX280 More Equal Less
The GTX280
is 2x faster than 8800 GT and as you can see even the 8800GT is significantly more than half a GTX280 in most of the specs, so taking that into account, specs wise the GTX280 did very well. And as a ump said it's specs wise where the HD5870 "dissapoints". The 9800 GTX simply destroys the 1/2 GTX280 specs wise, so it was not expected a two-fold increase in performance. The GTX285 on the other hand is almost twice as fast as 9800GTX, thanks to higher clocks:
http://www.techpowerup.com/reviews/HIS/HD_5770/30.html
9800GTX => GTX280
Pixel Fill Rate = 1.6x
Texture Fill Rate = 1x
Gflops = 1.3x
Performance at 1920x1200 = 1.7x according to Wizzard's review above.
Performance at 2560x1600 = 2x
HD4890 => HD5870
2x, 2x, 2x...
Performance 1920x1200 = 1.45x
Performance 2560x1600 = 1.53x
It's because of the above that has "dissapointed".