Gareth, my experience is "it depends", as unhelpful as that may be. What it depends UPON may be several things, including:
1) the source CAD that is used to create the neutral format
2) the target CAD or the use for which the file is intended
3) the kind and complexity of geometric elements included in the design (e.g., prismatic elements, splines, surfaces, etc.)

For the kind of work I do, which sometimes includes product design with some complex surfaces but not many "Class A" type surfaces, and the 3D CAD I'm importing into (SolidWorks) I find IGES usually comes in cleanly with few gaps and short edges that have to be healed (and which often WILL heal with Import Diagnostics). Some type designs, especially those with Class A surfaces, are better to save in STEP AP214 format instead of AP203, but I have seen SolidWorks have problems with those in the past (I haven't tried that lately and for the past few releases). Other CAD software may not have such problems with STEP AP214. I have noted in the past that Catia V5 is particularly problematic for exporting very complex surfaces to import into SolidWorks, and Pro/E sometimes is not much better. SolidWorks Import Diagnostics has gotten much better over the past few releases, though. I find that to heal up everything I sometimes have to try healing gaps first and then edges, or vice versa, instead of trying to heal up everything at one time. And sometimes the diagnostics will yield something which is actually a solid even if it still reports gaps and short edges. At that point it may be as well to leave it alone and not try to "improve" the import any further.

Sorry I can't give you a more definitive answer, but the best thing to do if you're not getting good results is to try both IGES and STEP and maybe Parasolid to boot. My mileage varies, and I presume yours will also.

Source

 

I like step because you can save an entire assembly and all of the parts are still separate parts and not one big fused part. But I have not used iges much, just for importing into ansys on a couple of occasions and it worked fine (for single parts).

 

For those that work in the 3D solid modeling world, STEP format is preferred as it brings the part into your 3D modeling software in solid format. If it is an assembly, most of the time the parts will come through in step format as individual parts that can be edited separately. IGES format always brings in parts as a surface model and assemblies will be one single part with multiple surfaces. IGES files require further processing inside of the 3D CAD software to convert them to solid models. For me personally, Step or Parasolid is the best way to go.

 

.igs can leave the component parts of an assembly seperate as well, in fact I opened up such an assembly this morning. I would speculate that if you've had problems with that, it's based on the CAD system you are importing in to.

 

I agree completely with Gopher. Personally, I've found iges to have less issues than step when importing and exporting between Solidworks, Rhino, and Maya. Between Rhino and Maya, obj seems to work best for me. But at the end of the day, it depends on the geometry and trying out different formats to see which works best for the application.

 

STEP data is the best, since it retain the Solide geometry even after translation, IGES good for sufarce data translation

 

I have to say, as an importer of many different parts from a wide range of software that IGES consistently gives me the MOST problems and issues. I mainly use Solidworks with Rhino as a back-up for really problematic files. It seems to me that the problem is often with the exporting software rather than the importing one but of course the luxury of getting the exporter to adjust their settings to produce the most useful export file is an uncommon one to say the least! For me Parasolid is the preferred exchange format, closely followed by STEP then ACIS (sat) with IGES as a format of last resort. I have always assumed that the only reason anyone uses IGES is because they have been in the field for a long time and it is the format with which they are most familiar.
Strangely, the general trend on this discussion seems to be broadly in agreement and yet IGES is still the format I am most often offered. It would be interesting to know the thoughts of IGES favouring individuals as to why it remains in common use.

 

I agree with Mike H. I use Solidworks and my preferences are, in order; native SW, Parasolid, STEP, IGES. I have also had reasonable success importing ProE native.
If possible I request more than one format but I cannot always communicate with the person who generated the original model.

 

That is a very valid point, ask for two differennt verions and a jpg or drawing of what it should look like !

Tony