Kyle,
Do you typically see new investors taking out FHA loans? A new investor with a solid credit score and excellent reserves and DTI ratio would be wise to go conventional. But to answer your question directly, I would say yes simply because FHA is a little more relaxed than conventional when it comes to multi unit properties/investment properties.
Or are there experienced investors also using them? Yes even experienced investors use them because of the amount of "good debt" they take on, they are still higher risk and may still only be able to qualify for FHA since it is government insured, but again every situation is different.
Also, how much would you estimate that the mortgage insurance raises your monthly payment? The monthly mortgage insurance or Up Front Mortgage Insurance that gets rolled in (UFMIP)? Monthly would vary depending on the interest rate and loan amount. UFMIP maybe brings the payment up $20-$30 per month but again also depends on the loan amount.
Or how much does it add on to your APR? This is a good question. Most people worry about the interest rate on the loan more than anything but they forget to focus on what the Annual Percentage Rate (APR) is. The APR is taking in all closing costs, all UFMIP, monthly MI, and interest rate all into account and re-amortizing the loan over 360 months (30 years) which would lead to a much higher APR. Granted if your pay the loan off early or a big chunk of principle the APR no longer applies unless you re-amortize it over the remainder number of years. Long answer short, it affects it a lot. Say you are given a 3.875% interest rate but with everything added in, your APR may look like 4.625%