In terms of latency, it doesn't compare to best-of-class PCI performance from RME or Lynx, but nothing much does. You just still can't beat the best PCI interfaces. But we're talking miniscule performance gains on a modern computer with PCI vs firewire... and honestly, we've passed the point of it being an issue if you have a current computer. CPU headroom being what it is now, in practice, this difference is not very noticeable. Ergonomics, workflow, features, sound quality, integration, stability, etc.... those carry as much or more weight with me now. That's why I've now felt totally comfortable using the MR for heavy VSTi work.... the performance issue is no longer the driving force for my decision.
In the realm of *firewire* interfaces, the MR holds its own, in the upper class for low-latency firewire performance on Win 7, in my experience compared to many other interfaces, including RME. But it's not the best there is... frankly, I had a MOTU unit in here with some very nice low-latency performance... yes, even on Windows. RME might use fewer resources from some tests I ran last year, but the last time I checked, RME's firewire driver didn't go down to 32 sample latency, which the MR does. That might have changed by now, it's been a while since I used RME firewire stuff. And BTW, I can't vouch for performance on OSX.
On an i7 class machine I can easily run the MR at 32 sample latency with Cubase with a TI-based firewire card, although I don't use that setting in production. Like most other firewire interfaces, the MR series appears to have an added safety buffer, but again, that's not an issue in practice.
I comfortably run the MR at 128 sample latency for fairly large projects all day long on a machine also configured with VE Pro, which streams to another machine over the network and back, and it is still very playable (to me... a drummer might disagree
). I'd probably feel comfortable at 64 sample latency on this machine, but 128 seems to be the magic sweet spot for my personal needs for most projects with enough headroom for lots of plugins, and I'm used to the latency for VSTi work. Your mileage may vary. I used to be sensitive to any delay around 128 samples and up, but I'm either getting older (and slower) or have just adapted.
I know all that's not scientific, but my suggestion is to try one out on your system and see what happens.
If you buy one, make sure you buy it from a place that will take it back if you don't get the latency/performance you want.
For me, there is only one other unit in this class, and that's RME (I utterly dislike the MOTU preamps, so MOTU is out, unfortunately), and whatever advantage I get out of RME (Totalmix and *possibly* *slightly* less system resource usage from the driver), is made up for in what I consider superior sound from the MR (I've never been an RME preamp fan either) and integration into Cubase... not to mention cost savings so I can buy other tools. Frankly, I feel the MR is the best bang for the buck on the market right now. I do love RME's Totalmix, but in the end, I decided on the MR for my Cubase rig and I've been really satisfied.
Hope that helps a little! Again, your experience may be totally different than mine. Good luck!