Ick. I've never liked any HSA custodian that was ever attached to an employer-sponsored plan that I have been a part of. (WEX, PayFlex, BenefitWallet, BenefitAccess, WageEquity, and some others I might be forgetting.) The often have fees, require quite high minimum amounts in cash (earning near 0% APR) before you can invest, have limited investment options, have cumbersome websites that make it difficult to invest, difficult to manage the amount you want deducted from payroll, etc.
I use Fidelity's HSA, and it is awesome.
Once per year, my employer deposits some funds into the HSA account (the one at the custodian that they selected. They essentially make a partial HSA contribution on my behalf.) After this happens, I immediately transfer the funds over to my Fidelity HSA. I opt to NOT deduct my own HSA contributions directly from payroll, and I instead do my own HSA contribution directly to Fidelity. I know that I miss out on "avoiding the FICA and Medicare taxes" on the amounts that I am contributing this way. However, since I have been exceeding the maximum FICA wage base amount anyway, the only extra taxes I'm paying is the 1.45% Medicare tax on the contributed amount. That's a very small price to pay for having better control over my investments, and not having to deal with the payroll-deduction-mess.