Just bought the AKG 451 today from RicherSounds (an audio retailer in the UK) and I'm loving them.
So far I'm using them connected to my Cambridge Audio DacMagic XS (USB DAC) connected to my Apple Mac. Sounds very smooth, not overly bass heavy.
Using the AKG 451 with my classic IPOD 160Gb, the headphones are very easy to drive producing a "full" sound which means the volume rarely goes above 30% when inside and 40% outside.
People have observed that the headphones are a bit bass heavy especially if you're accustomed or prefer to listening to neutral sounding headphones.
However, one short-coming in my opinion is that they offer poor noise isolation especially in noisy environments such as the Tube. Then again, what doesn't!
My other portable-to-go headphone is the AKG 518 which offer far better noise isolation but are a tad heavier (being circumaural headphone) and have an excessive clamping factor which make the AKG 518's tiresome after an forty minutes or so.
If you're looking for portable headphones which are easy to drive, fit comfortably and affordable, the AKG 451 are easily recommended.