I have one, and I contribute the maximum amount allowable and do not expect to have any leftover! But the key piece of information you’ll need to determine what your employer’s specific rules are on the annual limit you may contribute to the plan in pre-tax dollars. Employers generally limit the annual amount each employee may contribute in order to reduce their financial risk of pre-funding (the employer fronts the amount of money you will contribute over the course of the year), and each employer may set different limits. Pre-funding risk occurs should the employee leave or be terminated and thus no longer pay in to the plan, that means the employer does not recapture their pre-funding from the employee’s payroll deduction, which is why they set these limits. But these amounts can range, mine is something like $2,500/year (I don’t think that amount is extraordinary, but some employers may give a higher limit like $5,000).
Having said this, its very, very easy to rack up expenses which qualify. Every co-payment amount is eligible for reimbursement, as are out-of-pocket expenses for things like glucose tablets to treat lows, prescription glasses, etc. I figured that my prescription co-pays for insulin, test strips and doctors visits (my endo, primary care doctor, ophthalmologist, dermatologist, dentist, plus you can include any other specialist YOU might visit such as a gynecologist, etc.) alone would come close to the total, and I could easily meet any leftovers with eligible out-of-pocket expenses, but you may want to consider this relative to your employer’s maximum contribution amount.
The benefit is that your contributions are made with pre-tax dollars, meaning that it reduces the amount of your income which is considered taxable is reduced (thus reducing your taxes), so it benefits you to use such a plan if you have it available. But ask what the maximum contribution amount for your employer is, and do some quick calculations on all of your co-pays based on number of doctors visits and prescription refills you are likely to do each year. I agree that you may wish to be conservative the first year and adjust going forward, but the benefits are very real and can save you on tax liability!