Experienced opinions please on the differences (advantages / disadvantages) between telescopic and one piece rods.

This is not an "experienced opinion" as you requested. However, you're not going to get the sensitivity, responsiveness and overall quality with a telescopic rod as a one-piece, or two-piece, for that matter. I know of nobody who uses a telescopic EXCEPT under circumstances when they needed the telescoping feature for storage and transport. That said, a $5 rod from a junk shop will catch fish and so will any other type of rod.

Are you talking about a rod with a telescoping butt - like many flippin' rods?  Or a true telescoping rod - like an old car antenna.

I have one telescopic rod that I use for flippin'. The disadvantage would have to be for me, breaking it down to put it in my rod locker. It is 7'8" and my rod locker only holds 7' max length. :-[ I feel there may be a little sensitivity lost but not enough to matter because I use 50# braid on this rod. As you know braid is very sensitive. I really like a longer rod for flippin' and may in the near future upgrade to an 8' rod. I feel that if it is a quality stick, very little sensitivity will be lost. Rod makers have came a long way in the past 10 years or so.

advantage- storage.

Disadvantages:  weight, balance, minor sensitivity loss... and the biggest disadvantage is that they almost all break prematurely if they are used as a +T handled rod.   When collapsed, the inner edge of the butt section rubs the outer edge of the tip section and makes a perfect ring... which wears with time and that's where every one of my factory rods have broken.  I put a small protective wrap on the customs to eliminate that.

But since a new rod is cheaper than a boat with an 8' locker- I don't mind the +T handles.  ...lol

I am talking about a flipping type.