I've been a mostly faithful user of TurboTax, except for the year that it had weirdo copy protection built-in, when I used H&R TaxCut. I've already prepared my taxes this year with TurboTax, so I decided to download the free version of TaxAct and see how the numbers came out in comparison to TurboTax (and to double-check my work, since my taxes this year are more complex than in previous years).
In a nutshell, TurboTax had me getting a slightly bigger refund (~$150 more), but that's probably a result of my typing the numbers incorrectly into TaxAct, particularly for the minutae of my business expenses (although it appears that TaxAct may have screwed up its automatic handling of some foreign tax that was paid, on my behalf, by a mutual fund that I own). I'm sure that if I'd gotten everything in there the same way, the final number would have been the same.
In terms of the user interface, TaxAct is pretty annoying. For starters, TaxAct wants to save your tax data in C:\Program Files\2nd Story Software\TaxAct rather than the directory of your choice. Give me a break. TaxAct likewise nags you throughout the interview to spend the extra bucks for the Deluxe Version. TurboTax (I've got the "basic" version) generally leaves you alone. The "interview" style of both products is comparable, but TurboTax may be a little easier to deal with and zoom around. One noticable different is that TurboTax lets you see "my return" or "all forms", whereas TaxAct only lets you see every single form. TaxAct likewise seems to ask you more questions than you really need to answer. For example, I want to deduct some of the miles I drove with my car as a business expense. TurboTax was happy to do it for me without too much trouble. TaxAct started getting into all of these details about how I might be depreciating my car. I just want my standard $0.375/mile deduction, nothing fancy.
So, is TurboTax worth $30? You get Federal E-Filing for free (after annoying mail-in rebate), where TaxAct wants to charge you for the privilege. You get slight ease-of-use improvements. You get easy importing of data from Quicken. You get free downloading from your bank / stock broker (which TaxAct also wants to charge for). All of these things can save time, pariticularly if you've got a complex investment portfolio and don't want to spend all afternoon doing data entry.
In summary, I'd say that as your tax situation gets more complex, TurboTax starts looking better, and the price is pretty cheap. Still, it's hard to beat free, and TaxAct is filling out all of the same forms and is getting the right answers.