I use a rectangular cooler with a manifold as my base mash tun (5G batches typically and a couple of Parti-gyle batches making 10G total) and swear by it. No stuck sparges, no channeling (batch sparge). I have noticed slightly better heat retention in the round cooler I have added for a HLT. I do plan to use the HLT with a false bottom to make smaller batches, since the smaller footprint allows for extra grainbed depth for a small batch (2-3G).
Secondary: Oktoberfest, German Pilsner, Double IPA,
In Bottles: Lucknow IPA clone, Rough Rider Brown Ale clone,
John Harvard Imperial Stout clone, Hoppy Amber, Witch's Brew (Habanero and Smoked Corn Small Ale), Porter, Dunkleweizen, Dry Stout, Irish Red Ale, American Maple Wheat Ale, Black Wit, Belgian style Wit, Belgian Golden Strong Ale
Kegged: IPA, Saison, Hoppy Brown Ale