Since IB are based in the UK, you pay a little bit in banking fees every time you send money there. It is about CHF 12 regardless the amount.

Then there is the cost of real time market data:
http://interactivebrokers.com/en/p.php?f=marketData
Depending on the market you are trading. In many cases it is waived if you generate enough commissions.

Regarding tax - you get tax statement from them as with any broker. In Switzerland there is a distinction between a professional trader, and a non-professional trader. The tax authorities decide on your status based on factors such as the frequency of your trading and turnover. If they deem you professional, you pay income tax on profits, but you can also deduct commissions and other expenses (hardware etc.). If they decide that you are non-professional, there is 0% tax, counts as capital gains.
Once you start making significant sums, as with any other profession, best to get a good accountant/tax-advisor.