This is a personal decision with no right or wrong answer.

It depends on how much you value you a quiet(er) place to sit, free drinks (soft, at least), clean restrooms, getting boarding passes with no or minimal lines, a bit of snack food, internet access (maybe), newspapers to read and stuff like that. It then depends on how much you value $300 or so relative to those. (In the U.S., if you file Schedule C or have hit the 2% of AGI threshhold for employee business expenses, you can probably deduct the cost from your taxable income.)

For me, it's worth it, and I pay out of my own pocket. I don't travel every week but at least every other week on the average. I'm especially grateful for the lounge when a flight is delayed or I miss a connection. I don't mind 20 minutes in a gate waiting area, but four hours is a bit much for me.

Is it worth it for you? Only one person can answer that.