Simplest KVO using blocks. You don't have to use ReactiveCocoa to blockify KVO.

KVObserver.h

#import <Foundation/Foundation.h>

typedef void(^KVObservingBlock)(id value, NSDictionary *change);

@interface KVObserver : NSObject

- (instancetype)initWithBlock:(KVObservingBlock)block;

@end

KVObserver.m

#import "KVObserver.h"

@interface KVObserver ()

@property (nonatomic, readwrite, copy) KVObservingBlock block;

@end

@implementation KVObserver

- (instancetype)initWithBlock:(KVObservingBlock)block {
    self = [super init];
    if (self) {
        NSCParameterAssert(block != nil);
        self.block = block;
    }
    return self;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    id value = [object valueForKeyPath:keyPath];
    self.block(value, change);
}

@end

usage.m

self.titleObserver = [[KVObserver alloc] initWithBlock:^(NSString *title, NSDictionary *change) {
    NSLog(@"Doing it right! %@", title);
}];
[self addObserver:self.titleObserver forKeyPath:@"title" options:NSKeyValueObservingOptionInitial context:NULL];

// Later that day...
[self removeObserver:self.titleObserver forKeyPath:@"title"];


// Do it better.

The next thing you should do is -[KVObserver observeObject:keyPath:]. Default options (choose yours), no context. Weakly store object and copy key-path for later removing using -[KVObserver remove]. Optionaly store multiple pairs weakly using NSMapTable.

Also, to avoid strings, use @keypath macro:

#define keypath(object, path)     (((void)(NO && ((void)object.path, NO)), # path))

NSString *path = @keypath(self, title);

This will autocomplete the key-path, validate and refactor it (or at least will show error).