I have been a T-Mobile customer for about nine years and was a Sprint customer for over a year. According to PhoneScoop these two may soon become one here in the U.S. There has been talks before of T-Mobile USA being merged with Sprint, but it now seems these talks are getting more serious, which makes sense given that the 3rd and 4th largest U.S. carriers are both losing customers and having a tough time competing with iPhone carriers Verizon Wireless and AT&T.
Deutsche Telekom is the European parent of T-Mobile USA and according to Bloomberg Businessweek they are talking to Sprint about selling off the T-Mobile USA part of the company. Reportedly, there are issues around valuing T-Mobile USA so nothing may ever come of these talks, but it is interesting to think about how this could shake out.
T-Mobile USA is a GSM carrier (you use a SIM card with their phones) while Sprint is a CDMA carrier. I like GSM carriers myself since I can bring any phone onto the network by popping in a SIM card and have the most flexibility for phone purchases. T-Mobile USA uses HSPA+ for their "4G" network while Sprint uses WiMAX. The merging of the two would mean that the new company would have to figure out which technology to use for their network and I hope it would be the GSM one. LTE also uses SIM cards and is the future for fellow CDMA carrier Verizon Wireless so I imagine if this combined company was forward thinking they would go with the GSM/LTE option as well.
Both Sprint and T-Mobile USA have the lowest wireless plans and seem to be the most "open" with things like wireless hotspot connectivity. Neither carrier has an iOS device, but they do have Windows Phone 7, Symbian, Android, BlackBerry, and webOS devices. A combined company may be more appealing to Apple too and we could eventually see an iPhone on such a company as well.
I have been a long time T-Mobile customer due to the great coverage I get, excellent customer service, support for unlocked GSM devices, fast data speeds (now with fast 3G and HSPA+), and reasonable pricing plans. Sprint has very good pricing plans too and in recent years I have been happy with their customer service. It all depends on the technology a combined company may use because I think going all CDMA would not be good for us happy existing T-Mobile customers. I would hate for there to be just one GSM wireless carrier in the US, but then again when LTE is prevalent this may not matter as much.