Most cable companies offer a discount for a commitment, but will let you have no commitment for another $5/month or so. However, be aware that TIVO will hit you with a 1-2 year commitment, depending on the up front cost and the plan you choose. Usually the best deal is to shop craigslist or eBay for a used unit with lifetime service. You can also add lifetime on a new unit, but it will cost you an additional $400 upfront, as opposed to $20/mo for 2 years.
The TIVO premiere requires 1 cablecard, and that will usually require a truck roll from the cable company with a $40 cost (unless you get it as part of the initial install). It will also cost $3-5 a month for the card rental.
I really like the TIVO interface and online features, but be aware that you lose all interactive features available on the cable company's DVR. These include all video on demand channels. You will also probably need a switched digital video converter box. This is provided free by the cable company, but I have found it to be troublesome, and the cable companies don't understand this hardware. ˆt generally takes multiple service calls to get it all working, and then you need to reboot every week or so to avoid losing sdv channels.
It is worth those hassles for me, but these are things you should definitely consider.