If you are a dedicated transportation cyclist, this probably happens to you. It does to me, like clockwork, as the days start getting nice in the springtime.
It happened yesterday.
An acquaintance says, "I've decided I'm going to start riding to work. In fact I've already started - I've ridden a couple times this week. I'm wondering what kind of bike I should get... can you advise me?"
My advice to him - and I hope he takes it - was, "Get a good set of smooth-tread tires installed on that mountain bike of yours, and ride it for a year or maybe even two. Make sure that bike commuting is indeed for you, before investing a substantial amount of money into a newer bicycle."
Some smooth-treads and slime tubes would set him back $50 if he shops carefully.
If that goes well, he could add lights, and/or a rack, and/or fenders, for a few bucks more.
I hope he isn't thinking he NEEDS the latest leading-edge top-of-the-line urban assault bicycle. These days the bike stores are full of (full-price) SWEET road bikes of every configuration. And those salesmen, bless their hearts, would be happy to relieve my friend of $1000 or $1500 or more, in exchange for a bike that MIGHT end up on hooks in his garage after a month or two.
I've got another acquaintance who comes immediately to mind. He's a gadget freak. He had the same notion a few years back, about riding to work... it was convenient, since he's only maybe 3 miles away over flat terrain. He didn't ask for advice; he just went out and bought a nice full-suspension well-equipped brand-name mountain bike. That wasn't quite right, so he bought a super-lightweight road bike... the kind that only has about 8 bladed spokes on each wheel. (You can see I'm not an expert. I admire 'em in the bike store, but that's good enough for me.) And he got the candy - the cleated pedals and shoes, the computer, the titanium bottle cages. I think he paid $200 for his super-lightweight saddle, for cryin' out loud! And guess how the story ends? He drives to work every day and those two NICE bikes are on hooks in his garage, 360 or so days each year. (Nestled comfortably among many other dust-covered gadgets, I might add.) I imagine that if he got the "bug" again, those bikes wouldn't be adequate... they'd be trade-in material for newer and shinier and lighter-weight replacements.
If you want to be a bike OWNER, go ahead and get whatever appeals to you. If you want to be a bike RIDER, make sure that's what you want, before making a huge investment in bike ownership. (My newest bike is 8 years old.)