I haven't noticed this on my Apple devices but on my Windows 7 PC with iTunes 12.01.26 I recently began hearing ads every couple of songs. I pay for Match and ad-free iTunes Radio was a big selling point for me.
However, I was able to resolve this issue (so far) by doing the following:
Under the "Store" pull-down menu, select "Deauthorize this computer..."
Even though Match should be now disabled, if the option exists go ahead and turn off iTunes Match
Your computer should now not exist at all as far as iTunes is concerned.
Next, from the "Store" pull-down menu, select "Authorize this computer..." and sign in to your account
Even though you are now authorized for your iTunes account, you still need to select "Turn on iTunes Match" from the "Store" pull-down
At this point it will go through its normal matching process but you should immediately be able to listen to Radio and select from your prior saved stations.
From the moment I completed these tasks, I have been able to listen to iTunes Radio on my PC ad free. It's only been a few hours though, so no guarantee how long this solution will hold. I have an email in to iTunes support and will update on any changes or new info.