Highs: on my road bike on my longer commute: the long winding, rolling lane between Earsdon and Old Hartley on a fine evening, and seeing the whole coast from St Mary's Lighthouse down to Tynemouth glittering ahead; alternately, when I am on the Brompton on my shorter commute, overtaking guys on 'serious' bikes and seeing them struggle to stay with me!
Lows: None really except if for anyplace there's roadworks and lane restrictions and drivers try to push past you if you give them an inch...
I love commuting.