You're pretty much forced to use the BIOS for overclocking purposes, as - unlike on other high-end Asus boards such as the P7P55D Deluxe - no software utility is provided for this purpose. It's a shame as otherwise the software front is very well covered. Asus InstALL automatically installs all the drivers and utilities for you.
These utilities include Update, which lets you easily retrieve and install the latest BIOS revision from within Windows, PC Probe II and T. Probe for monitoring temperatures and voltages, and Fan Xpert, which can intelligently control all your fans according to temperature. The coolest thing about Fan Xpert is that it will assess the performance of whatever fan you're configuring to see how well it performs and assign fan-speed levels to it accordingly.
In terms of overclocking this board is pretty good. Even without dipping into advanced BIOS features such as CPU Differential Amplitude or Clock Skew, and without changing the voltage (leaving the Offset option to handle this aspect automatically), we easily managed to get all four cores of our of our Core i5 750 running at an astonishing 4.2GHz when in Turbo mode. This was achieved simply by raising the internal bus clock speed to 200, and even then we were only constrained from higher overclocks by the fan's inability to cope with the extra heat output. This also limited our Core i7 870 CPU, which would run stably at 4333MHz (with bus speed at 180) before ramping up to excessive heat levels.
As expected then we have an excellently-performing motherboard; the question is if at £184 it's worth the premium. For less money than this you can get the aforementioned P7P55D Deluxe, which garnered itself a Recommended Award when we reviewed it last month, and did slightly better at overclocking.
The Sabertooth 55i has a little more here and less there. On the positives side, it offers higher durability (though this is unlikely to be a significant factor for most people, as motherboards tend to survive far beyond their useful lifespan) and eSATA on its I/O. On the negatives side, there's one less SATA header, one LAN port instead of two, and no co-axial digital audio output. Of course the P7P55D Deluxe's unique TurboV remote and special overclocking features are also lacking, though in terms of raw overclocking performance there's not much between them.
Asus' Sabertooth 55i TUF is probably the toughest consumer motherboard around, combining rugged features with excellent layout and truly impressive performance. However, for most it probably won't be worth the premium over 'lesser' boards.