Yeah, TMO had some crazy good deals at the end of last year. I also switched from Sprint to TMO after moving from Sprint's "free" plan to whatever their normal (somewhat obscene) plan was for about a year.
On the TMO side, I got the buy 3 get 1 line for free Magenta promo, an insider account code/discount, and a $200 visa card per line. I think I'm paying about $112/month for four Magenta lines.
The thing is, I switched in the first place because TMO always shafted their existing customers and gave new port-ins great deals, so switching to a "good" Sprint deal allowed me to get a better deal going back to TMO.
I say "good" because it was $30+/month for 4 lines, but Sprint's network blows and the price it reset to was just nuts.