I think a lot of it has to do with the way that you use the dvr. Many of the people who claim to have no problems with the HR series have a pretty light usage model compared to how I use it.
That having been said, there are obviously issues with it. Every new release has a dozen or more bug fixes, along with the mysterious 'under the hood' that may represent another dozen or two fixes. I've personally experienced a large number of issues over the past 15 months with them, and when I've called Directv they've acknowledged these as known issues with the product. Over time and software releases, the problems come and go so its obviously not something wrong with my box.
I've owned a bunch of tivo's and a bunch of directv products. Prior to the DVR release, the directv products were solid as a rock. I must have owned about 10 receivers and never had a single problem with any of them, even ones that were in service for 6-7 years. Reliable and pretty speedy.
I've owned just about every model of tivo as well. Aside from very early initial problems with a few that turned out to be bad hard drives right out of the box, I've had very little trouble with them. I have a pair of series 1 units that are about 9 years old. I've replaced the hard drives in them with bigger ones, put quieter/faster fans in them, but they're mostly original. Response times are very good, although they're doing much less. My experience with the directivo boxes were that they were pretty slow, but they were doing a lot more. The tivo HD's are much faster than my HR20-100's in almost every way and the remote control response time is superior.
I've never really experienced any recording reliability issues with any tivos except the directivo's, and I was able to track that down to problems in the directv guide data stream. For units like the series 1 and tivo hd that take guide data directly from tivo? Maybe a handful of shows over almost a decade didnt record right. I've missed a good 35-40 shows (and thats just the ones I noticed) with the HR20-100's in just a little over a year. Frankly, I dont rely on it to successfully record my tv shows at a rate better than 95%.
Certainly more problems than most, but most people who own dvr's record 5-10 shows a week and watch most of their tv live. The box has never been plugged into a network connection. No esata. Not really heavy tv watchers.
I record around 400-450 shows per week. About 70-80% of those are cyclical recordings of childrens shows because the directv box has a pointless and quite useless implementation of "save until I delete". Both tuners are at it at the same time for quite a good piece of the day. My wife uses directv2pc quite a bit. Both boxes are on the network and do plenty of VOD. Both have big esata drives. Not quite the same usage model as the "average" user who "has no problems", or at least doesnt bother to report them if they do.
I also think theres a good range of varying tolerance between models and within models due to component variance and component tolerance. While directv and their proxies try very hard to assure everyone that all of their receivers are pretty much the same, they're not. Not even close. They use very different internal connections, some with tuners soldered into the motherboard and some on daughtercards plugged into slots. They use different cpu's and decoder chips. Different memory. I'm pretty sure the linux kernel in the HR20's is a different version from the other models. In fact, other than that they all use the same user interface and all perform the same functions, theres an awful lot of difference in the boxes and they SHOULD perform differently.
Within the same model, many different disk drives have been used. I see a number of different revisions and version levels of various components in the box, even within the same exact model number.
So in short, its quite unsurprising that many people using many different boxes implemented in many different ways and utilized quite differently with very different expectations of reliability and performance would find their experience to be different from anothers.
Whats surprising is that anyone would think that their experiences are the most common or most likely, or that few people experience problems, based on what is or isnt posted on an enthusiast internet forum.
As an analogy, most people use Windows. Most people have problems with Windows. Few people line up day in and day out to complain about each and every problem they have every day. They just reboot or close the program and run it again.
All that understood, its obvious that someone who journals their issues isnt the only one with the issues, nor that their experience is the norm. Its similarly certain that the lack of continuous broad based complaining is equally meaningless.
In places where product reviews are consolidated, its pretty common for the directv hr2x series dvr's to have a pretty piss poor rating. "Why, thats because only people with problems write reviews!" is the common explanation. Which is immediately belied by the millions of products with favorable overall reviews. If only people with problems wrote reviews, every product would have an overall one or two star rating.
They say that the first step to solving a problem is admitting that you have one...