Got a Circle F pipe on mine ('76-'78 large flange) & only because I had it sitting around. Most people bag them but I reckon it's great. The bike has a broad powerband (if you can say that about a 125), pulls hard and is not the slightest bit peaky. The engine is standard except for a port clean up, DG head, 30mm Mikuni with power jet and 14/52 gearing. Most of the comments I get are how fast the bike is and how "meaty" it sounds. I'm told it's hard to improve on the standard '76-'78 Honda production pipe though.
Cheers,
K