I assume you mean Daggerspell Mage, since Daggerspell Shaper is a druid prestige class.
While I have played an Arcane Trickster, I've yet to see a Daggerspell Mage in use, so I can't speak to how it will work in an actual game.
I will go ahead and say that you should check and make sure your DM allows Practiced Spellcaster before going with either of these classes.
Powerful is relative here. The AT is NOT a powerful prestige class, by any means. They're actually a lot like the Mystic Theurge - they can hold their own in a high level party, but getting there is painful. Very painful. If you decide on the AT, be aware that you will be fairly useless until about level 7, and the class doesn't really become rewarding until about level 10, depending on your build.
What the AT is absolutely best for is scouting/thieving. They have all the stealth skills, lots of spells to bolster them, and intelligence as a primary ability makes them wonders with traps, even before you factor in Detect and Dispel. All those early wizard levels do bad things to your skills, though, so again, you won't really start becoming an effective scout until level 10-11.
In a fight the AT has to cope with a terrible BAB, worse than a wizard's at some levels, a wretched fortitude save, and awful hitpoints, potentially worse than a comparable wizard's since you probably want to put your second best ability into dex rather than con. Melee is suicide. They can do some damage by combining sneak attack and ranged touch attacks, but this doesn't start working well until relatively high levels, when you have both enough sneak attack and enough spells to make it worth while.
The Daggerspell is obviously intended to be a melee version of the AT, from it's feat requirements. I'm honestly curious about how this would work out. They have only marginally better hitpoints, and a slightly better BAB, but suffer from the same basic problems as the AT. You would be completely dependant on spells, both to keep you alive and to make you an effective attacker. I'm a little leary of a class that can be completely disabled by a single Dispel.
They do get more skill points than the AT, but their skill list isn't as nice. Notably they lack search and disable device, which means your party will still need a trap monkey.
I can see two big things that might work in the Daggerspell Mage's favor. The first is that since you probably aren't the party scout, you can use all those utility spells an AT probably has prepared to buff yourself instead. That might make enough of a difference to keep you alive, with a sufficiently liberal use of False Life and Vampiric Touch on top of the normal defensive spells.
The second is the AT itself. If you could get a Daggerspell Mage to high enough level, the AT is the perfect class to round out your character with, and the requirements would be manageable. The opposite isn't true, since a high level AT almost certainly doesn't have the feats for Daggerspell Mage, and isn't really interested in losing another caster level.
If you do go with the Daggerspell Mage, come back and tells us how it goes.