If you lack experience with the series and have only breezed through a couple of charts and reviews, I can understand this viewpoint. If you purchase a card from the series or get the time to sit down and tinker with one, I think it's true merits come to light.
First off, most reviews of the high-end (58xx) cards were done on launch, and the drivers have substantially improved since then. If I had to give a rough estimate, I'd say there's about 5% increased performance across the board with the 5870, with upwards of 20% in some games (Crysis, for example). There was a similar situation when I had my GTX295, where performance against the 4870X2 was lackluster at launch, however, there was a review done a few months after launch that I would always link to as it showed the performance improvements from the newer drivers.
At high resolutions (like the 2560x1600 I play at, so this is the point of view I'm coming from), the 5870 stomps all over my GTX295, no questions asked. Not just in raw FPS, but also in smoothness and quality of gameplay. Also, I can keep cranking IQ and the FPS really don't suffer much, which is not something I could do with my GTX295. At lower resolutions, some of the older hardware might keep up, but definitely not on my 30". In addition to this, the 5870 is faster while using ~60% of the power. It also overclocks much better than my GTX295 ever could, while remaining much quieter. It also has support for the latest features (DX11, Eyefinity), which is also nice.
However, I think the best buy in the 5xxx series is the 5850. In an enthusiast's hands, it can do 1GHz on the core for only $280. One has to go into the <$100 market to find a better price/performance ratio, and it even bests anything offered by the 4xxx series at it's release in June 2008. While performance-wise, the 57xx series is the same or a bit slower than their 48xx counterparts, they also only use 55-60% of the power and overclock much better. I'd rather have that versatility and make up the performance with a quick clock change. I haven't used and therefore won't comment on the lower 5xxx series, and I love my 4350, but newer parts will always carry a price premium compared to last generation, sometimes out of proportion to their added functionality.
As far as the few glitches I've seen with the 5xxx series, the large cursor problem is a thing of the past (which, if it's really that much of a problem, you need to go outside more). Now I've built four separate systems, including my own, based on 5xxx series GPUs and have yet to see a grey screen crash that wasn't brought on by my own doing (usually an overclocking attempt). Have these problems actually been reported mostly by AIBs to AMD (due to increasing numbers of RMAs) or is it only showing up on enthusiast boards? Call me a cynic, but many times these issues develop from people not doing their homework and overclocking improperly. If you can't be bothered to do something thoroughly correct, don't bother.
In either case, my point is that NVIDIA is entering quite late into one hell of a competition. They need to have a stellar part just to stay competitive, nevermind progress.