From what I've read about them they were not at all good at their job and the only success in their legacy is dealing with the logistics of annexing neighbouring states.
Look up what they did in the Netherlands. They were very effective in gathering intelligence before the invasion, trained, supplied and provided information for the Brandenburgers who took vital bridges during the Invasion of May 1940, and managed to capture almost all Allied spies active in the Netherlands between 1940 and 1943, as part of Operation Northpole.
My source: Kingdom of the Netherlands during WW2, Loe de Jong
Their legacy is a rather muddled one as it turns out. From 1935-1944 the chief of the Abwehr, Wilhelm Canaris, never appears to be wholeheartedly committed to the Nazi cause and would eventually be executed for high treason.
Without a doubt, there were likely some diehard Nazi's in the Abwehr but when the chief of the organization is...less than enthusiastic, I cannot imagine getting many results. For instance, he contributed to Francos' decision to refuse German access to Spanish land by providing arguments that Franco would later utilize. He assisted several jews in fleeing the country and made several connections to M16.
Source - Bassett, Richard (2005). Hitler's Spy Chief: The Wilhelm Canaris Mystery.
German case officers were given a lot of free reign over their agents and operations. The control was much more decentralized than British Intelligence.
The careers of German case officers were also much more dependent on the success or failure of the agents each officer personally ran. This meant that even if they had private doubts about an agent (such as suspicions of being a double), they were obligated to keep supporting them as if nothing was wrong.
There were even case officers who nearly lost their lives based on their agent's operations. The case officer who ran double agent Eddie Chapman ("Zigzag" to the allies, "Fritz" to the Germans) was an Abwehr officer living the good life in occupied France. After Fritz was sent to the UK and disappeared for a few weeks, the case officer was transferred to the hell of the Eastern front in wintertime. After Fritz reappeared, the case officer was transferred back - but a shadow of his former self.
Whereas the credit or blame for successes and failures of British agents was shared throughout the agency. This allowed the British to run larger, more complicated operations such as the Double Cross and Operation Fortitude.
source: the Ben Macintyre books: Double Cross, Agent Zigzag, and Operation Mincemeat.