Ok, I think I can spread some light on ION since I have had it installed for a few weeks.  I have the Xt4 plan.  
Here is the deal.  The data speeds are in fact VERY fast, but hold your horses, there are negatives.  I live very close to my local CO and I am capable of recieving almost the entire 8Mbps service.  I have infact seen downloads speeds that are very high, sometimes as high as 630kbps.  Here is the problem though, all traffic is routed through the ION center in Kansas City.  I assume that this is due to the voice and other services traffic needing to hit a central router somewhere but I am not sure about this.  
Regardless my latency on most sites is in the 130 to 170 millisecond area.  This causes some serious speed issues due to the way that TCP traffic is handled.  (If you know better about tcp traffic than I do, please dont flame me on this, I am not a expert.  A clarification would be nice though.)  Anyway, this latency required some serious tweaking to my PC and my Mac TCP stacks and I am fairly content now that on large file downloads from fast servers I can typically get speeds of 380kbps - usual test downloads from Adobe.com and Apple.com.  I am still pretty much let down that I am getting half of my capable speed but not enough to complain because these speeds are more than adequate.  However, the latency does cause some sites to initially connect rather slowly in comparison.  
I have had ISDN, PacBell DSL and @Home.  Although the others tended to have better latency times, I HATE PACBELL and @Home was annoying when my neighbor would sign on and cut my speed severly.  
In regards to the LinkSys or other routers and the ION Hub.  The ION Hub has phone ports for your home lines (In my case 4 of them).  A DSL modem and (in my case again) 2 10/100 etherenet cards.  The cards are bridged and thus, only one of the ports needs to be plugged in to your LAN.  The other port will be bridged to your LAN and will give a sole computer connected to it full access to the LAN with no configuration necessary.  As for connecting your router, Sprint even suggests some Routers with FireWall protection on their site so they have no problem with you using one.  I am using a Apple Airport as a DHCP server for my home network, but any should work fine.
In regards to OS use, the ION system is using standard protocols and so any system can work on it.  In fact the Mac was just added as a supported platform in the last few days.  It wasn't supported when I got my install.  While the installers were at my home working, I allowed them to mess with my only supported computer, a PC running Win98 and I sat next to them and configured my Win2000 box and my Mac and they didn't care at all.  I suppose the comment about having to support a computer they list is the problem here.  The ION config software is web based and should have no problems with any Java compliant and secure browser.  And by the way, the ION techs told me that they will not be implementing any PPPoE now or in the furture  
 
Now for my phone lines, Sprint will take your existing phone numbers from your local telco and "Port" them to your Sprint ION account.  In other words my 3 Pacbell phone numbers are now Sprint ION numbers.  So I did get to keep my original phone numbers.  One of the lines however has to be a Sprint generated phone number.  So my 4th line is from Sprint.  For those of you considering 2 Line ION service if you have 2 lines already, you'll likely have to give up one of them, unless you go to the 4 line service.  
As for Apartments, Sprint told me that this has to do with the central wiring used in mutli unit buildings.  If your building has a line router in the basement, they arent ready for you yet, but they do infact plan on adding that service in the near future.
Additionally of note Sprint Tech Support is SUPERB!  I have never dealt with better support people with any product ever.  If there is a complaint, its that they call you too much.  One thing that you will never worry about with Sprint ION is "Are they gonna work on my problem?"  You'll know they are doing something.  The technology is new and sometimes figuring out the problem may be annoying, but you'll be very satisfied with the support you get.  This alone was enough to make me happy because this was the reason that I wanted to leave Pacbell in the first place.  Pacbell customer support is terrible and the people are down right RUDE.  Even though I know they don't care, I was thrilled that Sprint is giving them competition and they are losing my money.  (Sorry can't help but vent...  I should tell you sometime about my personal hell with that company... hehehe)
Anyway, back to the point of the forum...  My bills have changed as follows:  Pacbell 2 regular voice lines, with forwarding, remote forwarding, delayed forwarding, conference, and call waiting were approximatley $72.00 per month.  No voicemail, caller id or other services, not including local long distance or regular long distance.  Pacbell DSL line on line 3.  Standard Phone service on DSL line and occasional long distance faxing and a single IP address at the 1.54-384/128 service.  Cost $64 per month.  Combined my bills were $136 per month.  
Spring ION 4 line service.  750 long distance miuntes, unlimmited local toll calls, voice mail on all lines, caller id - all lines, forwarding - all lines, delayed forwarding - all lines.  And the Data line on top of that with 2 IPs (see above for speeds hehehe).  You can also reconfigure your services, switch your phone jacks, add or delete stuff, whatever with the software free of charge.  This is a great value.  Considering Pacbell would charge $6 per line for Voice mail and another $6 per line for Caller ID thats another $48, and then if you wanted Pacbell DSL 6Mbps service that would be another $130 extra you get the idea.  Pacbell equvalent services - $314.00 (before local tolls and long distance) vs Sprint ION $149 all included.
If you can use the services offered, Sprint ION in my opinion is the best deal you can get for phone and data services.  If you have more questions, post them to the board and I will try to get back to you.  Good luck and by the way be prepared for atleast a 30 - 60 day wait to get your install.  This isn't so much Sprint's problem as it is purposeful delays on the part of your local telco who has to do some rewiring to complete the install because they own the actual copper connected to your home.  They do what they can to keep you from switching, but they are required to do it by law, so they just do it slow.
<And now to catch my breath again...  ahhhhh>