I recently got both Sify and MTNL broadband connections working at home and contrary to what many have been saying, I've found Sify to be slightly faster and more importantly, consistent compared to MTNL. I've put together a little script that will make it easier for my mother to switch between mtnl and sify when I'm not around. It's basically simply upping one interface and downing the other along with copying resolv.conf for sify.
One issue with MTNL is that if the modem reboots, the interface gets disabled and does not get enabled automatically. I'm using USB (with DHCP, probably this is why) since I've already used up the two eth ports on my system (one for sify and one for my Celeron dabba). Hopefully I'll get this sorted out by this weekend.