I have been using this as my main sequencer for years; I suppose if I'd started with Cubase or Logic I'd be laughing at it, but what it can do it does nicely. I always felt intimidated by Logic's learning curve and frustrated by temperamental Cubase (and I'm neither a computer novice nor unwilling to RTFM)...but the simpler, budget sequencing programs were usually totally useless...any experience I had with Cakewalk was pretty unsatisfying too, although I haven't tried Sonar yet...
MIDI is as easy as it gets. Easy recording or piano-roll note entering (step-time too if you like that kind of thing), with easy drawing of controller (velocity, sustain, expression, RPN, NRPN, etc.) graphs- a big update from the previous version of DOP. I don't even miss the drum grid, but no groove quantization...so it keeps your rhythm ears sharp. I'm not too sure how well the sync works- maybe I'm missing something but it's not been a big deal. No chance of software synth plug-ins, but it will happily drive external stuff like Retro As-1, Reactor, Gigasampler, etc...and if you work more with soundfonts than outboard equipment, as i used to, you will be pretty happy with DOP.
Audio? Forget about plug-ins (real-time or otherwise) and the included effects are not often useful- export audio to soundforge or something for processing. But cutting and pasting bits of audiofiles is easy and precise (drum and bass heaven) and you can add a lot of audio tracks without any slowdowns or shutdowns or loss of audio/ MIDI sync. Occassionally the audio quality seemed a litle bit 'brittle-digital' as DOP translates the audio rather than working with the source wav file, but more often it sounded just fine.
Best point: easy and quick compared the big guys and pretty sophicated compared to the budget sequencers. Worst point: no expandability (real time effects/synths/plug-ins)...but anyway, DOP really does provide the quickest and least frustrating way to get your ideas down before the creative juices congeal, and there's plenty of tools to tweak the tracks once they're laid down. And it doesn't limit you too much, as long as you don't expect it to do a lot of the work for you.