I agree with MCV in as much as you need to eat all the calories or you will be deprived and crave things that will only make you feel more guilty/likely to fall off the wagon.
I started my weightloss (start weight 179lbs) with SlimFast/UltraSlim back in Nov last year and lost 11lbs before Christmas (168lbs). I then decided to have Christmas off (put some back on) and start Slim and Save in Jan (lost another 11lbs taking weight loss to 161lbs) but only lasted 12 days on that and I've not been able to get back into any kind of healthy eating plan since then :-(
I've started on UltraSlim again today weighing in at 163lbs and would dearly love to get to 140lbs. I know I can do it but I just need to get in the right place. Too much eating nice things with the children and the 'ahem' odd glass of wine!
You ask about long term but as I can see it if you were to stick to it for 3 months you could easily lose nearly 2 stone (2lb a week is 24lbs in 12 weeks, mid May) but make sure you have your snacks!
I like eating biscuits, chocolate and crisps so I factor then into my snacks. 2 biscuits are around 100 cals, quavers are 88 cal and a mini fudge bar is 64. I can have ALL of those if I want as my snacks and I don't feel deprived. I log everything on MFP and am true to myself (even when I'm naughty!). If I have a light dinner I can even squeeze in a small glass of wine. It's all about portion control.
If you want a buddy I'll set the challenge that BOTH of us will lose 24lbs byt May 17th? That will take me to goal and you soo much closer. I felt so much better about myself after the 1st 7lbs and it really spurs you on to lose more.
Good luck x