I've been riding to and from the Train station for at least 18 months now and its a 2.3km ride each way.
Began with a Haro BMX which being 6 foot wasn't great. Then "upgraded" to a K-Mart Cyclops thing - dual suspension, front cable disc, however that died when the crank(?) bearing failed and everything went loose!
Now though I've bought this off eBay
Its a Scott Aspect 45 (2009) - hard tail but dual hydraulic discs. Its incredible what a quality bike can do for you! I've got the receipts from the original bloke and he paid $1000 for it in August 2009 and I bought it 2 weeks ago for $175. It'd just an all round faster, smoother ride. There are bargains out there and I don't fully believe in needing to buy something brand new.
It's up to you as to what design you want - if you're primarily going to using it on the road then definitely go for a road bike but if you're like me and want a variety then I think something similar to what I have would be perfect. I ride to the station on the paths during the week (have to due to traffic) and then I like to smash a dirt trail on the weekends followed by a smooth road ride home.
As above as well - put a good high pressure in the tyres (I use 60PSI) and seat height is crucial! Go as high as you can!