This'll harm competition
This'll seriously harm competition in the US. At the national level, we have AT&T (GSM/UMTS) and Verizon Wireless (CDMA/EVDO) as kind of "tier 1" providers -- they are expensive as all hell, but provide nationwide coverage (Verizon moreso though). Sprint (CDMA) and T-Mobile (GSM) are lower cost but have much more limited coverage, relying much more heavily on roaming for coverage. (The iDen portion of Sprint, Nextel, is kind of a wild-card.. it really doesn't fit in well with Sprint as a whole, it's a totally seperate network, and there is not even push-to-talk working between iDen and CDMA phones yet last I heard. iDen *was* considered to be on it's way out, but last I heard it was actually making Sprint money while the CDMA part was not even close...) Then there's "everyone else" -- regional and local providers, MetroPCS, Cricket ,etc. (MetroPCS, Cricket, cover just a few cities but have cheap unlimited phone service). A merger of Sprint and T-Mobile reduces it to 2 high-cost carriers, 1 lower-cost carrier and "everyone else".
Hopefully if they do merge they ditch Sprint's customer support -- I've talked to several people who liked the service, liked the phone, LOVED the cheap plans, but as soon as they had to call in just couldn't get anything done, even if it was just some minor billing problem.. they'd get more and more frustrated until they switched to another carrier.