I don't know about trading LLCs. However I do know about LLCs in general (I have one).
Obligatory talk to a lawyer:
Be very careful with muddied accounting. If you're trading personal capital, you can't "use" the LLC to fund yourself. All the money in the LLC needs to be the company's and you pay yourself some wage after. Otherwise you pierce the veil and the protection provided by the LLC is rendered moot.
Make sure to set up a very clear and very well written operating agreement as further protection. Most importantly know what LLC laws and regulations apply for your state.
Also make sure you guarantee that the LLC will protect you in the event of a wipe out. It's possible just trading is enough to pierce the veil. You'll also need to be aware of export taxes, etc if you trade futures or futures backed ETFs.
I doubt you need a consultation to create an LLC. You can use a simple service and just consult a lawyer on your operating agreement and stuff. However, there may be a better corporate structure for a trader - so perhaps advising ($400 seems cheap when it could cost you million to be wrong) is a good idea.
You'll also want to look into business insurance. Some places won't insure risky businesses so expect to pay an absurd amount for insurance if you're a leveraged trader. It will be worth it.
As for LLCs in general. Yes, the tax savings can be significant. You must be careful though and follow the IRS rules. It's easy to overstep and get a nasty audit.