T-Mobile on the other hand is having greater success with their CEO and all of the "uncarrier" tactics they are implementing. T-Mobile is now focusing on upgrading the last of their own cell sites to LTE to expand footprint in the rural parts of the country but they still have a ways to go. And that's not to say T-Mobile's existing LTE network isn't great, it's fantastic (if you can actually get it).
AT&T is out of new deployable spectrum so their LTE network is slowing down in congested areas. They are deploying tens of thousands of small cell sites to make their network more dense to help with this problem. So say a tower has 100 people sharing 20mhz (10x10) of LTE bandwidth at the same time. If there are now three small cell sites placed away from that tower, each additional site has 20mhz of spectrum to use. So there would then be ~25 on that main tower, and ~25 on each additional small cell site.
Verizon's original 700mhz LTE network is very congested in a lot of areas, but instead of deploying as many small sites as AT&T, they instead bought that very large chunk of AWS spectrum from cable companies and have it deployed to the majority of their cell sites. In major cities this is a full 40mhz chunk of spectrum (20x20) which theoretically can net you up to 150Mb/s if you have the right device that can use band 4. They also have started re-farming a small portion of their PCS spectrum (10mhz total) in some testing areas (san francisco especially). They will need to densify their network eventually but they sure have done a great job at buying an ample amount of time.
I personally feel Sprint's current slump all comes down to the Clearwire/WiMax decision. Both Sprint and Verizon were in the same situation when it came to deploy their choices of 4G technologies. They both operated CDMA/EVDO 3G networks which with the revision they were using maxed out around 3.1Mb/s. AT&T was in the middle of deploying their HSPA/HSPA+, what they now call "4G" (not to be confused with LTE). Verizon and Sprint did not have the option of this transition period of significantly higher speeds that was not LTE or WiMax. Surprisingly Verizon got this right. They obtained a nationwide chunk of 700mhz block c (band 13) spectrum for their initial LTE network they started deploying in early 2011. This is the same spectrum that has the open access rules attached to it that made them back down on throttling on their LTE network. The sheer speed at which they deployed this LTE spectrum was just ridiculously fast. Right now they have 99% of their entire 3G footprint covered with that layer of LTE while AT&T's national map is still nowhere close to Verizon in terms of overall LTE deployment covering their entire legacy networks.
Meanwhile Sprint went ahead with WiMax and we all know how that unfolded. I hope they can recover as 4 national carriers is always going to be better than 3 competition wise, but I just don't see it happening for potentially years from now.
In terms of network, Verizon got it 100% right the entire time. AT&T struggled in ~2009 with the iPhone and recovered but is feeling the effects of congestion but they are countering with tons of small cell sites. Verizon has congestion on the band 13 LTE deployment, but band 4 is helping with that. T-Mobile's LTE deployments are great, but they are still focusing on expanding into more rural areas. Sprint is working on their Spark network, and where you can get it, it's good, but they just are moving so damn slow I don't know if it will be fast enough in the long run.