You should be looking for only two things in an investment plan:
The ability to approximate the total market with the provided funds
Minimization of expenses
TSP allows you to approximate the total market fairly well. You could perhaps do slightly better on your own, but it would take a decent amount of effort and any benefit would be negligible in the long run.
You will not find a fund in the private sector that comes close to TSP expense ratios. Even low maintenance funds in the private sector that are widely considered to be 'great' in terms of expense ratios are ~3 times more expensive to the investor than TSP (for example: the SPY ETF). On top of that, TSP trades are free so re-balancing your account costs you nothing.
Take whatever you have in TSP and find a mix of the available funds to suit your risk tolerance (or use the lifecycle funds), and then come back every 6 months to re-balance the funds back to your ideal mix. In between that time, forget about it. At retirement your TSP account will have more cash in it than had you attempted a comparable strategy with any other private sector fund.
There is no good monetary argument for removing your funds from TSP (assuming you're still investing on a 'long term' timeline). If some buddy of yours tries to convince you that TSP is shit, make sure you educate yourself before you make the irreversible decision to withdraw from the Program. Once you pull your funds, you can't get back in when 5 years down the road you realize your mistake.
I have only one complaint with TSP, and really I'm probably just being protected from myself. I'd like to be able to sit everything in my account on the sideline as cash if I want. Financially it makes no sense, but I still find myself wanting the security of knowing I could if I wanted to. However, it's not enough to pass up the opportunity to participate in TSP. I can move my civilian sector investments to cash if I feel the need, and again, it's a stupid move anyway.