Walk the streets, halls and open offices of Silicon Valley, and you’ll notice many tech workers wearing them: Allbirds’s Wool Runner sneakers. But the merino wool-made sneaker, which debuted to much fanfare in March 2016, has a problem: they simply don’t last.

“Wearing Allbirds is like wearing slippers everywhere,” said one startup founder, who likes the comfort they offer because of the materials used and relative lack of structure. “But wearing them every day comes at a cost: I chew through a pair at a rate of every 4-6 weeks, after which I have to buy another.”

Jonathan Yaffe, CEO and co-founder of software maker AnyRoad, echoed that sentiment.

“The liners inside get worn down really quickly,” Yaffe agreed.

At $95 a pair, the Allbirds Wool Runners don’t come cheap. In fact, they’re nearly twice the price of some tried-and-true Converse Shoes. But they’ve gained a foothold among the technorati, a hoodie-loving culture that prizes comfort over looks on any given day and has the disposable income to spend on it.

Founded by Tim Brown and Joey Zwillinger in 2015, the San Francisco-based Allbirds was created with the mission of innovating upon everyday footwear, a segment of the shoe market Brown and Zwillinger viewed as crusty and stagnant.

Nike, for example, would just put its signature swoosh on subpar shoes made overseas, Brown complained to Business Insider last May.

So Brown and Zwillinger, who have raised at least $10 million in venture capital-backed funding to date, emerged with the Wool Runner, a sneaker employing a proprietary wool and nylon blend they say helps minimize foot odor and wicks away moisture while making sure your feet don’t get too hot. Even better: should the sneaker get dirty or smelly, cleaning it is a matter of simply tossing it in the washer and air-drying. As a result, the sneaker that calls itself the “world’s most comfortable shoe” exceeded first-year revenue projections by five-fold, Allbirds told Yahoo Finance.

But the apparent downside to that proprietary wool and nylon blend seems to be a serious one. Half a dozen members of the tech community Yahoo Finance spoke to for the purposes of this story reported a love-hate relationship with their Allbirds Wool Runner and find themselves purchasing new replacement pairs every two to three months.

While the Wool Runners may be the most comfortable shoes they’ve ever worn, they just don’t last. It’s an issue I learned with a pair of my own after wearing them nearly every day for just six weeks. While they’re perfectly comfortable — the “world’s most comfortable shoe” moniker isn’t wrong — the lack of structure means I now look like I’m traipsing around San Francisco wearing dark, lumpy, misshapen mounds of wool on my feet.