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Question: I have Comcast cable TV with a DVR box, high speed internet and phone service. If I buy a TIVO Premiere XL4 what would that replace and anything else I have to buy and any monthly fees involved?  Rich
Document: The pictures are better than comcast DVR box I had, the cost is high because I signed up for life time cost $499.99 and monthly $19.99. I Wished they have added the power button with Standby Mode on the unit for easy access to rather then hidden menu that would required the remote control.
This is my third TiVo. I have a Series 3 XL, a Premiere XL and the new Premiere Elite. I absolutely would not buy any other brand of DVR and I was more than happy to get rid of the Comcast DVR box that we were using in our family room. I LOVE the capabilities of the new Elite box. We have many shows that overlap and particularly with the cable channels that we watch (History, Discovery, TLC, etc.)
Had TIVO since 2005. Got tired of not being able to find anything decent to watch when I wanted some down time. We recently upgraded the TV, so it was time to upgrade the DVR. Tried our cable company's DVR, but am totally spoiled with TIVO. After a quick phone call with the cable company and a few pointed questions with disappointing answers, I immediately told my wife I was getting a new TIVO.
I had the TIVO DirectTV Box that i bought back in 2004 and we moved to WA from PHX in 2007 and DirecTV did not want us to use the box they wanted us to use there DVR Box which was Crap and it kept breaking down and messing up the recordings or not recording the full show or it would but it would not play them. DirecTV could not fix our issue so i dropped them like a bad habit and got Comcast Cable and for xmas we bought the XL4 and we love it.
This will also allow you to stream pics and music through your TiVo so you can hear it on your TV or your Home Theater. You can set shows to record from your phone or your computer. TiVo will even suggest shows for you. There's even a couple of games  to play on the TiVo. I can go on and on. My whole family is hooked. My only complaint is the increase in the monthly service fee of $19.99, which I only pay for the Premiere.
I've been a long time user of TiVo products since the first series years ago. I have typically found that the products continuously high quality. I almost always buy lifetime service and recently done so again with the Premiere XL4.This product has met almost all my expectations. The seemingly only issue that has risen has been with the service outages that appear to on the TiVo service side. Also there are some minor latency with the HD menus within the interface.
You have to continue paying for the box indefinitely or you will receive no functionality even though you own the hardware. Even previously recorded shows will not work although the website claims they will. When you call customer service they will tell you that the only way you can use your box for anything is to continue paying monthly. How is this even Legal? With all the cable DVR's coming out I hope Tivo goes out of business!! !
In particular, I don't like the the ribbon of suggestions that appear at the top of the My Shows menu. I would love a way to turn it off. It always seems as if I accidentally navigate to it while trying to do something else.Overall, though, you can't go wrong with the Premiere Elite. It's simply the best, easiest to use DVR out there. And one final note:  we had a terrible time getting the crappy Comcast cablecard to work.
This long time Tivo customer is xtremely disappointed. ****UPDATE JUNE 2013****One would have thought that a company like TiVo, which makes DVRs and nothing else, facing renewed stiff competition from cable company DVRs, would have done something in the way of a software update to speed up the incredibly, annoyingly, unacceptably SLOW as molasses XL4, but they apparently have no problem letting their customers suffer on a daily basis navigating an interface that is easily surpassed by the movement of a sloth.
It was a very surprising and delightful experience to set this puppy up. Only problem I had was returning my comcast equipment and getting my cable card to work. I'm now up and running, I have wireless network service, netflix, amazon movies and hulu. I purchased a second remote with the slide out keyboard. Great partner for my Sony internet/3-d/ led/ 46 "  1080P hd TV. All is great in my house.
Answer: The Tivo would replace your Comcast DVR.  You will have to go into Comcast to exchange the Comcast DVR for a CableCard that goes into the back of the Tivo.  Comcast will generally credit you for the cost of the cablecard when you return your original box so no extra monthly fees from them.  You will however need to pay the monthly Tivo subscription fee.  I found it worthwile since the Comcast box can only record 2 shows at a time and the interface on the their guide is terrible.  The only issue you may have is Xfinity On-Demand.  The Tivo's only support a few cities so far.  I am in Chicago (one of their largest markets) and we still do not have on-demand on the Tivo.  I end up using my Xbox for on-demand until Comcast rolls out in Chicago.