I've used older versions of Rosegarden a bit, haven't tried latest (v1.2.3). I'm a big Linux/open source fan but compared to Mackie Tracktion 2 (the main audio/MIDI program I use in Windows), Ableton Live, Sony Acid, Fruity Loops and other Win/Mac sequencing programs I've tried the Rosegarden interface is pretty darn obtuse. Assigning soft synths to channels is a pain, and I'm not always sure if the documentation is updated with each new release.
If you haven't already, check out the "support" and "resources" sections at
http://www.rosegardenmusic.com/
If you're interested in doing multimedia on Linux I strongly recommend that you use a distribution designed for that purpose. It's not too difficult to run several versions of Linux on separate partitions using the GRUB bootloader. If you're a Linux/UNIX expert you can compile your own kernels and programs from source but this takes time away from music making, especially if you want to update regularly.
I suggest Planet CCRMA
http://ccrma.stanford.edu/planetccrma/software/ which is without a doubt the best maintained/supported Linux multimedia distro I've found. Planet CCRMA mailing lists are very informative for both hardware and software concerns.
Also worth checking out is
Invalid Link Removed which has just released version 2.0 of their dynebolic bootable live CD-ROM distro.
Hope this helps--good luck!