I successfully upgraded several RB1000, RB450G, RB750, RB433AH, RB493, RB493AH, RB532A, RB192, x86 devices to RouterOS 4.1. There were a broad range of technologies used in these devices, IPSec, OSPF, wireless, PPTP, L2TP, switch, bridge, QoS, sophisticated firewall, NAT, mangle, L2 firewall, to name the most important. None of the devices failed during upgrade, license updates were smooth as well.
Everything is ok so far, I consider RouterOS 4.1 the best release ever.
Thanks MikroTikls, and keep up the good work!
PS. BGP wasn’t mentioned because I didn’t upgrade BGP routers, I’d prefer to be on site if anything fails 
I had several problems with 4.0 upgrades… namely loosing WAN interface settings everything set to default and interface disabled)
Was that fixed in 4.1 or is it because I went from 3.20 to 4.0 or because these boards all had extra-channels licenses or anyone have any other ideas? Its a bad time to lose all that info and have to reconfigure remotely.
Other than that, yeah seems to be good operational release.
Scott
Any 3.x less than 3.27 I upgraded first to 3.30, then I updated license, then upgraded directly to 4.1. In each case I took extreme precautions, that is made backup and did /export.
I also observed that sometimes in case of problematic devices, which I mean behaving oddly during the upgrade, the problem may be with some sort of I don’t know, I call it ‘garbage’, that gathered in the device. If there’s a problem, usually Netinstall followed by restoration of the backup ‘heals’ the device and it doesn’t cause any problems in the future. YMMV.