Cal Poly is my alma mater, class of '07. I loved all 4 years I spent there. Please note, I did a bit of commuting from home to class this one summer, and took a bike on the trail through Strawberry Canyon once, but overall, my experience biking through the city and the neighboring environs is pretty limited.

The campus itself is not flat. However, I remember a fairly good amount of bike parking (especially along Via Carta which bisects the science and engineering buildings). The core of campus where you'll be spending the majority of time as an engineering student is fairly compact and easy to navigate. I look back and wish I was into biking back then.

San Luis Obispo, on the other hand, I imagine to be a joy to ride in. Downtown is also fairly compact, and flat. The main stretches are a mix of one- and two-way streets, so it may take some time to get used to the layout. The streets are not the widest, but traffic in SLO is generally not too congested. However, if you're into longer rides, SLO has GORGEOUS backroads, e.g. Foothill to Los Osos Valley Road out to Los Osos/Morro Bay. See also: beachside biking at Pismo Beach, 10 miles away.

Unfortunately, I cannot comment on how prevalent bike theft is around campus and SLO.

Let me know if you have any more questions. Good luck on your decision!