I find that mirror lock-up is only useful in the 1/2 to 1/8 range. Any slower shutter, and the mirror slap isn't long enough to matter. Any shorter, and the exposure is too fast to be effected. Composing with the LCD is ungainly, for now. I'd imagine the newest crop of DSLRs (olympus, at least) with p&s style live-view make it a lot easier.