I've handled about seven or eight different Marttiinis as part of my small eBay enterprise, and I've never failed to be impressed with any of them. In terms of steel quality and blade geometry (including the famous Scandinavian grind, with one convex bevel on the edge), even the cheapest one I've seen (Lumberjack model) is top notch -- I marvel at how inexpensive they are given the blade quality.
As with many knifemakers who sell at different price points, the price differences usually come down to the non-blade parts of the knife. I keep a Lumberjack (4" fixed blade) at home for general utility use; it has a nice carbon steel blade but a rather cheap-looking (but sturdy) hardwood grip (for under $20, I can't complain).
If you want a utility knife, the Lumberjack is the way to go (note it has no fingerguard). For a few bucks more, the Marttiini hunting knives with aluminum bolsters and rubber grips are solid and come with nice bullhide deep pocket sheaths in a variety of sizes and blade styles.
The top-of-the-line Marttiinis ($75.00 and up) usually feature birchwood grips and (if they have them) bronze fingerguards. In this price range, I recommend the Marttiini Lynx, which comes in different blade lengths; some come with metal fingureguards, and some have finger choils carved into the grip, which is a good middle ground.