I like that it has many different modules for simulating various processes; this makes using any one of them very easy to learn, use & understand! It also allows you input parameters from other software packages (such as Excel) so your model can be easily transferred between programs if needed or desired by yourself/the client at some point in time down stream - which would otherwise require manual transfer via text files etc..
The only thing i dislike about dwsim itself are its interface issues when running certain models with large number inputs such high numbers being displayed incorrectly causing errors within calculations made through excel formulas referencing these values!! This problem however does not occur often but sometimes needs attention quickly before things get out hand / wrong calculation results obtained during testing phase leading into production stage may need immediate correction resulting wastage due loss sales revenues!