No experience with Inspiron and decent experience with just a few Vostros so far, but Latitude is normally what I'd recommend people get when they're unsure just because the enire range is truly 'road ready' as Dell says. They are designed to take a beating, and feature quality build and components. They're not quite Thinkpads (which are overrated in any case) but Dell have definitely done the homework for building robust notebooks for business use. I like the D630 (although it's not the lightest out there) as an all-purpose machine and if you're looking for something very portable the D430 with the Mediabase is an excellent choice. However these Latitude machines don't have things which you equate with an all-rounder with a fun aspect - things like webcam or a gaming-friendly graphics processor. These are for the Serious Internets
I think the Vostro range is pitched right for small businesses and single-person entities who want something which can tackle a bit of fun as well as the serious stuff. The Vostro 1500 which I was testing for potential use in a project is a well priced and well rounded machine. Nothing jumps out at you - like most things Dell - and build quality is fair - around Apple Macbook levels - but it does definitely get the work done, as well as (I said before) some fun. The black-only finish may be restrictive, but since when has a black laptop been totally passe?
If there's one thing I definitely have to say, it's don't skimp on the support (this goes for Inspiron, Vostro or Latitude although the Latitudes have uplifted support). Then the likelihood is high that you'll have a great experience with Dell. A lot of the complaints I see about Dell are people who pay buy the cheapest machine possible, pay nothing for additional support and gripe when things go wrong.