PTC's Pro/Engineer was the very first 3D CAD program I've heard of back in beginning of 2002.
I was working in design office as Design Engineer. It was my first official employment within mechanical engineering company.
Company itself didn't have much CAD resources, designers were practically "fighting" who's gonna sit and work with only one relatively modern PC in the office at that time and design in AutoCAD. Most of the job we were still doing using good old drawing-board.
My chief then gave me a job to try to figure out kinematics of bus' windscreen wiper mechanism on a drawing board. I made a mechanism layout on a board according to existing hand drawing, but when it came to figuring out the paths of points of interests (we were interested in kinematics and dynamics of chosen points of mechanism because we wanted to make some design changes in one of the members of mechanism), first troubles appeared.
The paths of points weren't simple, key element had a sort of swaying movement, and it was very difficult to resolve the mechanism's behaviour. I've made a simple cardboard mock-up of physical model in suitable scale, and managed to "discover" path of certain points after few tryouts.
But then I was beginning to wonder, there must be some piece of software in 21st century that would make these kind of jobs easier for designers, package that would give you a chance to animate mechanism and to study its behaviour...luckily, I've had a colleague who has heard about software that could help us to simulate mechanisms - yes, he was heard of Pro/Engineer.
The first version of Pro/E I've ever had a chance to work with was Pro/E 2000i.
It was my first step in 3D CAD world and it was like the whole new world arose in front of me.
One of the hardest CAD package to learn at a time, with strange user interface, inhospitable, tailored for Unix users (I've discovered this later) , with a drop-down list of infinitely long and unclear (for beginner) commands from a menu, with few icons, difficult to get used to for a windows user, with strict and stringent workflow and exact order of moves to get the job done.
It had a very strong internal logic, it demanded of user to know how to plan his next moves in advance, and it learned you to be very disciplined and drilled in order to achieve your design goals. Most of beginner users had a trouble to perform even a simple circular pattern of a feature in earlier versions of Pro/E.
But it was also very powerful package, equiped with strong CAM and simulation modules, very stable and customizable piece of software.
It is considered as one of three top-range CAD/CAM/CAE/PLM solutions.
Maybe because it was so hard to learn Pro/Engineer when you compare it with other CAD packages, the pleasure and fullfilment when you acquire and conquer new skills, internal logic of program and new moves with such sort of software was the biggest of them all.
Just a simple example: in earlier versions of Pro/E you couldn't leave a sketch environment if the sketch wasn't fully defined and constrainted.
And usually you would first have to make a choice of feature, and then after that to sketch suitable profile for that feature. There was a strict order of moves once.
But there was also one good think about this "struggle" - when you learn to work with Pro/E, all other CAD packages were much easier to adopt.
Any Pro/E user could easily move from it to any other 3D CAD package-the opposite is very difficult. I imagine that Catia or SolidWorks user would have a real hard time to switch to Pro/E.
Nowadays as a design engineer you have similar flexibilities within Pro/E like in all other CAD packages. You can make an unconstrained sketch and you don't even have to use it for any feature, if you later decide you don't need it.
Newer versions of Pro/Engineer ("Wildfire" 2, 3, 4 and soon WF5) have undergone major changes in user interface and now Pro/Engineer has more acceptable UI and its complete environment is more tailored and suited for average windows user.
Although still a bit behind of Catia's and SolidWorks' UIs for example, Pro/E manages nowadays to keep in step with other CAD packages and retains its wide user base.
And Pro/E certainly deserves it.
Some examples of my early works in Pro/E you can find on this page.