A few answers, just for discussion
1) I have a Premier Elite/XL4. Got it not long after the Elite came out. It failed, and was later replaced by an XL4. Also have (still) an HD and a newer Premier. The 4 tuner is clearly less snappy than any of the other boxes. Bringing up "now playing" has the list populate far slower, starting to play a show is slower, the guide is slower, etc. It's not terrible, but it is measurably slower - meaning that from a very very consistent perspective, you could ALWAYS measure how the 4 tuner is demonstrably slower than 2 tuner counterparts. It is certainly possible that it's not just the extra tuners, but is also the extra recorded content for which the device is simply not efficient in indexing and dealing with.
2) Lifetime vs monthly. There are many reasons why you can debate this. Assuming that current units still relevant (still usable and therefore having value) and you maintain documentation, reselling a unit that you have lifetime on and getting a new one is most likely the best financial decision. However, times are changing and the list of "non support" in terms of features is IMHO changing the game. An HD for example to me, given what Verizon is doing, the lack of stream, the inability to use a mini, seriously reduces its resales value. So, I'm not convinced that the arguments of yesterday are valid moving forward. You would have to narrow your purchase and resale window very narrowly and hope that Tivo does not effectively desupport your existing hardware in order for that argument to be rock solid. For many, that's not worth the risk. For many others, it certainly is.
3) Since the HD came out, I've had 3 HDs, now 2 Elite/XL4s and a "regular" premier 2 tuner. Every single one was plugged in to a good UPS from the time it was installed (contrary to what Tivo support said I should do). Every single unit failed within 12 months and was replaced, with the exception of the newest "regular" premier. In contrast, not a single other "entertainment" device in my home has failed during that period with the exception of a Toshiba DLP TV. So, until such time as we can truly have a disaster recovery option, I won't be putting all my eggs in one basket. As for whether anything you lose can be replaced otherwise, the answer to that is a resounding NO. Far too much content is simply not available on hulu, etc - not to mention the idea of "slightly" time shifting live sports like NFL games. The problem here is that if you assume everything is available via hulu, etc, then frankly Tivo loses a great deal of its value proposition - as then no device specific subscription or equipment is required any longer. Tivo is only as valuable as it is because of the inability to get much of that content - at least for some of us. Like I've said otherwise, if I can't get that security that what is to be recorded does get recorded and stays that way, then Tivo or anything else similar just lost its primary value proposition for me at least. Not beating on Tivo here. That is exactly the reason I moved from an Adelphia/Comcast SA8300 solution to Tivo back in '07.
As far as the advantage of having a single now playing list that unifies all of the devices, frankly Tivo should already be doing that rather than forcing a new hardware platform to get what is already available. At least online, we should already be able to manage all of our Tivo "universe" in one spot rather than the very clunky device by device method. At least that's my position. There is no technical reason why that is not possible. Not sure how what exists has been implemented, but Tivo completely controls the API, and with web services, there is no real obstacle that I can fathom.