We use it, and it is adequate. But, as an engineer who is fluent in both AutoCAD and Terrmodel, in my opinion TMOD is far more awkward to use and not at all conducive to engineering-type drafting. Some shortcomings follow:
It is hard to use the Ortho grid; it requires many more steps to perform the same functions; hatches can't be previewed or editted; dynaviews (TMOD's attempt at Viewports) aren't interactive; dimensions are not edittable; default settings are complicated to change; you can not imbed OLEs (Word or Excel); there is no drag and drop insertion; you must hit Tab instead of Enter or Spacebar to progress to the next step in a command (therefore requiring two hands at all times, or endless pointing and clicking until your wrist aches); labeling and digitizing are so unbelievably complex that you will dread doing either; snapping an object to another object with an elevation does not give the new object that elevation (like in ACAD), so you have to separately assign elevations to each new object; text windows are not adjustable, the only way to get multiline text into a thinner area is to manually insert a return where you want each line to end...great, until you want to reshape the text area; you can't scale an object realtime, you have to blindly type a number, then see if it fits, then undo and repeat until it fits; when trying to match properties, you have to specify each property to change, instead of the 'paintbrush' Windows and ACAD offer; you can only have one TMOD file opened at a time for some unfathomable reason; blocks are so hard use that I actually put a block in a new drawing then tmx it in...this is OK, except you can't control the scale, insertion point or whether it come in exploded, it comes in exactly as it was when the tmx file was made; TMOD doesn't remember the last plot configuation, so when making say twenty identical plots, you have to redefine the window each time; there is no polar array (TMOD command is matrix) and it is confusing...I always matrix in the reverse direction and to a wrong spacing (you can't pick either realtime or preview, so again it is matrix, undo and repeat until you like it); it doesn't remember your last offset distance...so when grading a site at 3:1, you must specify 3 unit offset with each new contour; it wouldn't give you an area unless certain criteria are met, it will also not tell you why...I usually just draw the whole object over instead of trying to figure it out; if you remove a block you forever get a message that it couldn't find EACH COPY OF THAT SAME BLOCK unless YOU recreate it, at each redraw or zoom... in big files this really slows down your computer; list is limited to certain types of objects and only certain attributes can be listed. And even though TMOD theoretically exports to ACAD, you will (trust me, from first-hand experience) spend unbelievable amounts of time making your data usable to anyone trying to open it in ACAD (which, of course is almost the entire rest of the engineering, surveying and drafting world).
I will bet your engineers are likely to 1) already be fluent in AutoCAD and 2) resist replacement with Terramodel. I would move carefully before recommending across-the-board replacement.