Isabella von Carstein...
...brings a subtle variation to your army.* She is defensive in nature and a useful support character for a more powerful Vampire tasked with killing enemy characters.* Special characters are more often than not cheaper than customising a standard hero or lord character. This is especially the case with some of the Vampire special characters (most notably Mannfred von Carstein). Special characters also often give you access to powers and items that you might not otherwise be able to attain. The only real downside to special characters is that some Lord level characters can be prohibitively expensive in points (even if they are good value per point spent). Another biproduct of selecting special characters is that they can, sometimes, limit your tactics a bit.
The Vampire Countess
brings with her a unique magic item that offers you the ability to restore lost wounds to Vampires (including Isabella). The item in question is the...Blood Chalice of Bathori
- which restores 1 previously suffered wound to Isabella or another Vampire in the same unit at the start of each Vampire magic phase.This can be quite handy, especially against enemies equipped with special targetting abilities enabling them to single out your Vampires (eg. the Hochland Long Rifle)
Isabella also posseses the Beguille Vampiric Power. This makes her more likely to wound enemies because she can reroll failed to wound rolls if the target does not pass a leadership test with a -3 modifier (note: only 1 enemy model in base to base contact can be selected per combat phase).
Isabella is also a level 1 Vampire wizard (normal in all respects for a Vampire spellcaster with no hidden extras - sadly)
Isabella von Carstein comes at a cost. Not her points value, which is quite low given her abilities, but her profile.
She has a lower weapon skill (-2), strength (-1) and attacks (-1) than a standard Vampire Hero. The attacks modifier is offset a little by her Beguille attribute but there is no doubt that she is not as an effective killer as your average Vampire.
Is Isabella von Carstein worth using ?
I think she probably is worth using in the same unit as another Vampire who is likely to spend time in combat and who you want to protect.
Or against enemies who can pick out your Vampires with special targetted ranged weaponry.
Isabella costs 90 pts which is a reasonable price to pay for a potentially useful 'life saving' character in my opinion.