Only make the full $15K into the TSP if you know you are not going to need the money until retirement. We've had a number of people here think they can put it in the TSP, and then decide later to pull it out for whatever reasons (need to pay bills, want to buy a new car, etc, etc, etc) and don't understand the potential tax liability, including early withdrawal penalty, if they wanted it out before they hit retirement age.
That said, TSP is a great investment for the long haul.
Finally, take a look at options with a Roth account. If you are in a low tax bracket, and expect to be in a higher tax bracket later in life when you are ready to take the money out, then a Roth can make good sense for you. Under Roth, you invest taxable money, and get it out at the end without paying taxes on the gains. Under a traditional 401(k) or TSP, you don't pay taxes on the way in, you pay as if it is taxable income at the distant end.
Most folks roll their outside 401(k)s INTO the TSP, because it has low costs, gives them good access, and is easy to manage. Yearly costs are tiny compared to any outside 401(k). The downside is the limited number of options to put it into- i.e. no gold fund, no sector specific funds, etc.
Your choice.
Study, and make an informed decision as to what is best for your situation. Talk to a financial planner too- not one that sells products, necessarily, but one that can look at your situation, and offer you choices.
Personally, I have talked to USAA- they have a free financial planner available to do an initial counseling, and then also can give you a "for a fee" counseling session where it is all laid out for you. USAA is available to all current military members, and is worth looking at.
