The original five manablades -that is to say, the first five created- were known collectivly as the Fingers of the Witchslayer. All had slightly different powers, but they had three things in common. They were all +3 keen, vorpal weapons. They were all encoded to specific individuals. And they all ignored any form of magical armour. Mage armour, for example, was ignored. This is still one of the most common powers of a manablade.
Mana blades are quite often keyed to specific individuals, or more commonly specific types of people. A good deal of the manablades made today are keyed only to humans, elves, or dwarves... these are the only races that practice arcanotech, and it's typicly a saftey measure to make sure the blade is never used against it's creators. Keyed manablades wont work unless specific conditions are met... most commonly, being held by a specific individual or type of individual. If someone who doesn't meet the conditions attempts to use the blade, it just doesn't work. The blade won't form. A few are booby trapped, most commonly with a sleep spell set to go off if a non-keyed individual picks up the blade. A select few (the blades that a few rulers have, for example) are made to teleport to a specific area in such an event. Removing the key from a manablade is almost impossible, it's woven into the enchantment from the start. It requires almost totaly rebuilding the blade, which looses any existing enchantments. A few (those keyed to individuals) are often set up to go "blank" if not used for a certain amount of time... normaly in years. Thus, if a blade keyed to a ranger hasn't been used in 10 years (for example) the key "resets"... the first person to pick it up is now keyed to it.
The Oaken Council gives a manablade to all of the Paladins and Gaurdians that hit the 5th level of a prestige class I designed. All of these are +4 keen, vorpal, Ironpass and Soulpass blades. Ironpass and Soulpass are enchantments that can only be applied to manablades. Normaly they won't work together, but the oaken council holds secret a method to get around that problem. Ironpass blades ignore unenchanted metal. They simply pass through it and cause no damage. This has it's own downsides, mind you... first of all, it works on metal only... wooden or bone armor would still work. Second... Hope you don't intend to parry

. Besides, enchanted metal, or adamantite/mithril (both of which are considered naturaly magical in my campagin) stops them just fine. Soulpass blades won't cause damage to a living thing with no soul... basicly, only sentients. Thus, you can't kill trees, or deer, or what not. It will harm undead, though... they aren't living anymore.
Also note... manablades (Or any arcanotech, for that matter) are never intelligent.
As for Arcanoaugmentation, there are a number of downsides depending on the procedure.
First of all, they are subject to such things as antimagic fields. While dispelling won't work, there, a disjunction spell would. This is a major turn off... While it doesn't matter to people who, say, lost their arm ANYWAYS, people are less willing to get one cut off if they know their replacement isn't as reliable as the old one.
Second, in many cases the augmentation involves loosing an old body part... gemeyes, for example, cost you your real eyes, while limbs are obvious. And in my campagin world that actualy is important... regenerating body parts isn't easy. Not totaly impossible, but not easy. Way outside the range of possible for the "average joe".
There are others, of course... worse ones.
Take the internal mana battery. Not only is the implantation very unpleasent and risky, there are downsides to the device. First of all, it's a vulnerabilty. If you are hit with a spell you have stored in the device, you have no saving throw against it. Saving throws are more difficult for spells of the same SCHOOL as a spell you have stored. And I hope you never plan to cast spells YOURSELF... you won't be, once you get that thing installed. It screws up your ability to channel mana too bad.
So, with that in mind, who in their right mind would use one? Assassins (not the class, the proffession) sometimes use them. A few fighters use them, those that don't expect to fight magic users as part of their daily line of work.
As for possibilities... there are a lot. Arcanotech wings, gemeyes that grant permanent abilities like see invisible or darkvision, replacement limbs that are sometimes better than the original, and internal mana batteries are only some of the things that have been done. A rogue in the party I DM has a strange one... It's a magicly fired crossbow (Basicly, it's just a long steel tube that holds one crossbow bolt, and launches with a command word) implanted in his right forarm. It's a last ditch manuver, but he can raise his hand up (like you were pressing it flat against a wall sorta) and speak the command word and the arrow is fired... ripping through his skin and towards the target. He takes some damage, of course... and he has to reload it before he gets magical healing (or else he has to injure himself AGAIN), but it has saved his bacon once. A villan in my campagin is HEAVILY augmented... his entire body is laced with mana-laden crystals, mithril wire, rune-carved adamantite plates, tattoos made with blood-ink, and a few brands. It's a very... visible... augmentation. It's all part of the same augmentation, though. It basicly gives him a form of Wildshape, despite the fact he has no magical ability himself. Of course, it has it's downsides... Intense exposure to magic can sometimes trigger a random shapeshift. And the randomly triggered ones aren't under his control... he can't just will them away. He has to wait for them to go away. Which isn't always quick. As well, he found he can't shift while he is wounded too badly... The system doesn't seem to recognise the target if it is messed up too badly. If he suffers enough wounds, he had to be healed or wait to heal before he can shift. If he's in an animal form, he has to wait... how many clerics randomly cast heal on animals?
Basicly, downsides for nanoaugmentation are more or less unique... since so many augmentations are themselves unique, it's often unknown what the downside will be untill someone works up the nerve to try it.