I'm not an expert on ustn but I do use it. The SS2 version has Real Drawing libraries so it might provide better fidelity than older versions when converting DGN to DWG. Depending on which version you have you can use the EXPORT command from the file menu to convert DGN to DWG, or you can just use SAVEAS. I'm not sure which method offers the best fidelity but I don't think either will be 100% since ustn has more capabilities than ACAD with regard to linetypes, references and other features and vice versa for some features in ACAD. Also sheets may not translate to conventional ACAD layouts if they were not set up properly for that purpose.
In Microstation v8i, SAVEAS .dwg usually works pretty well for me. Make sure you use the correct units when saving (Feet vs. Survey Feet) or your site might shift by a few feet. Also, using Windows True Type fonts instead of Microstation fonts helps.
The conversion mentioned by Neil will work very well. You also have the ability in SS2 to have the linetypes be written out as Autocad linetypes and translate MicroStation fonts to Autocad shx files. Using Export or save as will give you the same functionality. All the setings are under the dwg options (bottom right of the saveas or export dialog). It allows you to use the level symbology settings to translate if you want. It is fairly flexible. You then just need to hunt down the shx/lin files it creates and you will be good to go.
One thing to keep in mind most of our clients generate linetypes at a scale (so let's say they are developed for 40 scale plans). So on the acad side you may end up needing to mess with LTSCALE/PSLTSCALE/MSLTSCALE to be happy with the way those files work in Autocad. It also may end up making working with native Acad linestyles and Microstation linetypes in the same file a challenge.
A final note (really I promise this is it). Since Microstation does still generate some linetypes that are not supported by Autodesk I suggest that you check the check box that says "Drop unsupported Linestyles to Components" this way the "picture" still looks the same. Or you use the remap utility in MicroStation to map the lines to some other supported style.
We made some changes to Civil 3D 2011(using an API from Bentley) that have be added to AutoCAD 2012 (and Civil 3D 2012) that when a DGN file is opened, the linetypes in the Microstation resource file (rsc) are recreated in AutoCAD.
I am curious about how complex linetypes in Microstation might be translated to Autocad linetypes. I am no expert on Autocad linetypes but It is my understanding that Autocad linetypes are incapable of mimicing some of Microstation's capabilities. For example I have a Microstation linestyle that includes a curb & parking stalls when I stretch out a line. How does the new Autocad Translator handle these tyes of linestyles? It would be great if we could get this style to work in Autocad.
The linetype engine has some additional features for reading RSC files from Microstation and creating a linetype that can replacate the graphics inside of AutoCAD. At this time these are "special" linetypes that you can't edit or create using the AutoCAD UI. You will need to make sure that the location of the RSC file is in the AutoCAD path for the conversion to work.
We tested this on a good number of Microstation lintypes and it is working well. I love to here how your linetypes work.